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Mechanics  Dept . 


Library 


MECHANICAL  STOKERS 


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PUBLISHERS     OF     BOOKS      F  O  P^ 

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s 

I 


MECHANICAL  STOKERS 

INCLUDING  THE 

THEOEY  OF  COMBUSTION  OF  COAL 


BY 

JOSEPH  G.  WORKER,  B.S. 

PRESIDENT  OF  THE  STOKER  MANUFACTURERS'  ASSOCIATION 

OF  THE   UNITED   STATES,   MEMBER  OF  AMERICAN 

SOCIETY  OF  MECHANICAL  ENGINEERS 

AND 

THOMAS  A.  PEEBLES,  B.S. 

MEMBER  OF  AMERICAN  SOCIETY  OF  MECHANICAL  ENGINEERS 


FIRST  EDITION 
SECOND  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:  370  SEVENTH  AVENUE 

LONDON:  6  &  8  BOUVERIE  ST.,  E.  C.  4 

1922 


Engineering 
Library 


COPYRIGHT,  1922,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


PRINTED  IN  THE    UNITED   STATES    OF   AMERICA 


PREFACE 

There  are  few,  if  any,  books  on  Mechanical  Stokers,  which  are 
available  for  young  engineers  to  study  in  preparation  for  combus- 
tion or  stoker  work.  Many  books  have  been  published  on 
boilers,  furnaces  and  power  plant  equipment  treated  as  a  whole, 
but  none  treating  stokers  as  separate  units.  Nor  have  any  of 
these  outlined  the  best  modern  practice  in  stoker  installation 
and  use. 

There  seems  to  be  a  demand  for  such  a  book;  treating  on  com- 
bustion as  it  applies  specifically  to  stoker  work,  with  something 
definite  about  mechanical  stokers  and  their  applications,  as  well 
as  the  factors  affecting  their  selection  for  differing  conditions 
and  widely  differing  fuels. 

This  book  is  presented  to  fill  this  need.  It  is  a  record  of  first- 
hand knowledge  of  combustion  and  stoker  work  gained  in  years  of 
theoretical  and  practical  experience.  The  endeavor  throughout 
is  to  give  reliable  unbiased  opinions  and  facts  from  actual  field 
experience  in  the  design,  installation  and  operation  of  stokers. 

The  Authors  feel  that  there  are  great  possibilities  in  a  book 
of  this  character;  something  which  will  cover  the  entire  stoker 
industry.  With  this  thought  in  mind,  constructive  criticism 
and  suggestions  are  invited,  to  the  end  that  new  ideas  may  be 
incorporated  when  a  revision  is  found  necessary. 

JOSEPH  G.  WORKER. 

PITTSBURGH,  PA.,  THOMAS  A.  PEEBLES. 

February,  1922. 


vn 


581216 


CONTENTS 

PAGE 
PREFACE .       v v 

CHAPTER  I 

COMBUSTION — As  APPLIED  TO  STOKER  WORK 1 

Principles  of  Combustion. 

Elements — Carbon,  Hydrogen,  Oxygen,  etc. 
Air  Supplied  for  Combustion. 

Weight  of  Dry  Chimney  Gases  per  Pound  of  Carbon. 

Products  of  Combustion. 
Temperature  of  Combustion. 
Temperature  of  Ftirnace. 
Efficiency  of  Combustion. 

Heat  Balance. 

CHAPTER  II 

MECHANICAL  STOKERS  AND  THEIR  DEVELOPMENT 16 

Smoke  Prevention  Contrivances. 
Mechanical  Feeding  of  Coal  to  Furnaces. 

Early  Forms  of  Traveling  Grate,   Overfeed  and  Underfeed 

Stokers. 
Development  of  Mechanical  Stokers. 

Reason  for  Use  of  Stokers. 
Present  Types  of  Stokers  in  the  United  States. 
Description  and  Present  Design  of  all  Types  of  Mechanical  Stokers. 

CHAPTER  III 

COAL  AND  COAL-PRODUCING  FIELDS  OF  THE  UNITED  STATES     .       .       .80 
Coal  Definitions. 

General. 

Ash  and  Moisture  Free  Coal. 

Clean  and  Dirty  Coal. 

Size,  Kind  and  Grade  of  Coal. 

Caking  Coal. 

Clinker. 
Use  of  Coals. 

Percentage  of  Total  Tonnage  used  by  Industries. 

Tonnage  used  in  Manufacturing  Different  Articles. 

World's  Coal  Reserve. 

World's  Production  of  Coal. 

Coal  Production  in  United  States. 

Coal-producing  States  and  Percentage  of  Total  Production, 
ix 


X  CONTENTS 

PAGE 

Compositions  of  Coals. 
Proximate  Analysis. 
Ultimate  Analysis. 
Heating  Value. 
Classification  of  Coals. 

U.  S.  Geological  Survey  Classification. 
Anthracite. 
Semi-anthracite. 
Semi-bituminous . 
Bituminous. 
Sub-bituminous. 
Lignite. 
Kent's  Classification. 

Table  of  Coals. 
Commercial  Stoker  Classification. 

Eastern   Coals — West   Virginia,    Virginia,    Maryland   and 

Eastern  Pennsylvania. 
Pittsburgh  Coals — Western  Pennsylvania,  Ohio  and  Eastern 

Kentucky. 
Middle  West  Coals — Michigan,  Illinois,  Indiana,  Iowa  and 

Missouri. 

Eastern  Kentucky,  Tennessee  and  Alabama  Coals. 
Texas,  Oklahoma  and  Arkansas  Coals. 
Colorado  Coals. 

Washington,  Oregon  and  Wyoming  Coals. 
North  Dakota  Lignite. 
Coke  Breeze. 
Anthracite  Culm. 
Bone  Coal. 
Coal  used  by  Central  Station  Plants  in  United  States. 

CHAPTER  IV 

COMBUSTION  CHARACTERISTICS  OF  COAL  AND  SELECTION  OF  SUITABLE 

STOKER  EQUIPMENT 106 

Adaptability  of  Various  Types  of  Stokers  for  Burning: 

Eastern  Coals — West  Virginia,  Virginia  and  Maryland. 

Pittsburgh  and  Ohio  Coals. 

Michigan  Coal. 

Illinois,  Indiana,  Iowa  and  Missouri  Coals. 

Eastern  Kentucky,  Tennessee  and  Alabama  Coals. 

Texas,  Oklahoma  and  Arkansas  Coals. 

Colorado  Coal. 

Washington,  Oregon  and  Wyoming  Coals. 

Dakota  Lignite. 

Anthracite  Culm. 

Coke  Breeze. 

Bone  Coal, 


CONTENTS  xi 

PAGE 
CHAPTER   V 

DRAFT .       .   129 

Definitions — Natural,  Forced  and  Induced  Draft. 
How  Draft  is  Produced. 
Natural  Draft  Performance. 

Chimney  Capacity,  etc. 

Formula  for  Determining  Friction  Loss  in  Chimneys. 

Illustration  for  use  of  Curves  of  Chimney  Sizes. 
Draft  Losses. 

Through  Fuel  Bed  and  Grate. 

Through  Boiler. 

Pressure  Required  to  Create  Velocity  of  Gases  Leaving  the 
Boiler. 

Damper  Losses. 

Breeching  Losses. 

Friction  Loss  in  Chimney  and  Breechings. 

Method  for  Analyzing  Draft  Losses  through  Boiler  and  Breechings. 
Induced  Draft. 

Fans  Used  for  Boiler  Room  Purposes. 
Forced  Draft. 

Stoker  Fan  Selection. 


CHAPTER  VI 

FACTORS  AFFECTING  SELECTION  OF  STOKER  EQUIPMENT     .       .       .       .154 
Factors  Effecting  Selection  of  Suitable  Stoker  Equipment. 
Load  Conditions. 

Efficiency  vs.  Coal  Burned  by  Underfeed  Stokers. 

Efficiency  vs.  Coal  Burned  by  Overfeed  Stokers. 

Efficiency  vs.  Coal  Burned  by  Chain  Grate  Stokers. 
Coal  Conditions. 
Draft  Conditions. 

Pressure  in  Wind  Box  Underfeed  Stoker. 
Draft  in  Furnace  Overfeed  Stokers. 
Draft  in  Furnace  Chain  Grate  Stokers. 
Application  Characteristics. 

Chain  Grate  Stokers. 

Sidefeed  Stokers. 

Frontfeed  Stokers. 

Underfeed  Stokers. 

Single  Retort  Underfeed  Stokers. 
Furnace  Design. 

Brickwork  and  Arches  for  Underfeed,  Overfeed  and  Chain  Grate 

Stokers. 
Location  of  Observation  Doors  in  Furnaces. 


xii  CONTENTS 

PAGK 
Clinkering  of  Coal  and  Formation  on  Side  Walls  of  Furnaces. 

Air  Boxes  in  Side  Walls. 

Air  in  Side  Walls. 

Fire-brick  Blocks. 

Side  Plates. 

Water  Backs  and  Exhaust  Steam. 
Air  Over  Fire. 

Mixture  of  Gases  in  Furnace. 
Ash-pit  Construction. 
Draft  Effect  on  Brickwork. 
Flow  of  Heat  through  Furnace  Walls. 

CHAPTER  VII 

STOKER  EQUIPMENT  OF  MODERN  STEAM-POWER  STATIONS  .       .       .       .   186 
Edison  Electric  Illuminating  Co.,  Boston,  Mass. 
United  Electric  Light  &  Power  Co.,  New  York,  N.  Y. 
Public  Service  Electric  Co.,  Newark,  N.  J. 
Philadelphia  Electric  Co.,  Philadelphia,  Pa. 
Consolidated  Gas  &  Electric  Co.,  Baltimore,  Md. 
Buffalo  General  Electric  Co.,  Buffalo,  N.  Y. 
Cleveland  Electric  Illuminating  Co.,  Cleveland,  Ohio. 
Duquesne  Light  Company,  Pittsburgh,  Pa. 
West  Perm  Power  Co.,  Pittsburgh,  Pa. 
American  Gas  &  Electric  Co.,  Windsor,  West  Va. 
Detroit  Edison  Company,  Detroit,  Michigan. 
Union  Gas  &  Electric  Co.,  Cincinnati,  Ohio. 
Merchants  Heat  &  Light  Co.,  Indianapolis,  Ind. 
Union  Electric  Light  &  Power  Co.,  St.  Louis,  Mo. 
Commonwealth  Edison  Co.,  Chicago,  111. 
Minneapolis  General  Electric  Co.,  Minneapolis,  Minn. 
Denver  Gas  &  Electric  Company,  Denver,  Colorado. 

CHAPTER  VIII 

APPLICATION  OF  STOKERS — DETERMINATION  OF  SIZE        .       .       .  '     .  211 
Range  of  Efficient  Combustion  Rates. 

Peak  and  Uniform  Loads. 
Size  of  Stoker  for  Typical  Case. 
Minimum  Combustion  Rates. 
Continuous  Combustion  Rates. 
Proportioning  Stokers  for  Old  Boilers. 
Grate  Area  of  Stokers — How  Determined. 

Overfeed,  Underfeed  and  Chain  Grates. 

Determination  of  Correct  Size  of  Stoker  for  a  Given  Set  of  Condi- 
tions. 

Dimension  of  Stokers. 
Capacity  of  Coal  Hoppers. 


CONTENTS  xiii 

PAO» 
CHAPTER  IX 

INSTALLATION  OF  STOKERS — SPECIFICATIONS — CONTRACTS — GUARANTEES 

— BOILER-ROOM  LOG    ......       . 226 

Improvements  Possible  in  the  Installation  of  Boilers  and  Stokers. 

Items  to  be  Decided  when  Installing  Stokers. 

Cost  of  Installations. 

Stoker  Engineering  Data  Required  to  Study  Stoker  Installations. 
Typical  Specifications  for  Installation  of  Stoker  Equipment. 
Stoker  Guarantees. 
Stoker  Contract  and  Proposal  Forms. 
Stoker  Specifications  and  Materials  Furnished. 
Boiler-room  Log. 

Installation  Data. 

Labor  Data,  Boiler  and  Engine-room. 

Maintenance  Data. 

Coal  Data. 

Oil  Data. 

Boilers  in  Service. 

Electrical  Output. 

Water  Data. 

Temperature  Data. 

Pressure  Data. 

Flue  Gas  Data. 

Routine  Service. 

Smoke  Data. 

Heat  Balance. 

Cost  of  Power  Production. 

INDEX   ,  .  253 


MECHANICAL   STOKERS 


CHAPTER  I 
PRINCIPLES  OF  COMBUSTION 

A  knowledge  of  some  of  the  fundamental  principles  of 
chemistry  is  necessary  for  the  engineer  who  deals  with  combus- 
tion problems.  These  principles  when  considered  in  connection 
with  mechanical  stokers  are  confined  to  the  burning  of  various 
grades  of  coal,  lignite  and  coke.  The  chemistry  of  some  of  the 
steps  in  the  combustion  of  these  fuels  is  quite  complicated, 
especially  when  considering  the  volatile  contents,  but  a 
thorough  knowledge  of  this  phase  of  the  combustion  process 
is  not  necessary  for  an  understanding  of  the  engineer's  prob- 
lems. 

A  working  knowledge  of  gas  analysis  apparatus  and  espe- 
cially a  chemist's  appreciation  of  the  necessity  for  care  and 
accuracy,  with  the  ability  to  make  the  necessary  calculations 
and  draw  correct  conclusions  from  the  results,  is  all  the 
engineer  requires. 

The  principal  elements  encountered  in  combustion  problems 
are,  carbon,  hydrogen,  oxygen,  nitrogen,  and  sulphur. 

Carbon. — Carbon  is  found  in  solid  fuels  in  two  forms.  In 
the  solid  state  it  is  known  as  fixed  carbon  because  it  remains 
unchanged  when  the  fuel  is  heated  to  a  temperature  sufficient 
to  drive  off  the  moisture  and  volatile  contents.  In  combina- 
tion with  hydrogen  it  forms  the  combustible  volatile,  and  also 
occurs  in  small  quantities  in  combination  with  oxygen. 

Hydrogen. — Hydrogen  is  found  in  fuels  in  combination  with 
both  carbon  and  oxygen.  The  hydrogen-carbon  or  hydro-carbon 
content  is  an  important  part  of  solid  fuels  both  with  regard 


2  MECHANICAL  STOKERS 

to  heating  value  and  to  the  manner  in  which  the  fuel  will  burn. 
The  caking  of  certain  fuels  during  the  early  stages  of  the  com- 
bustion process  is  due  to  the  action  of  certain  hydro-carbon 
compounds  which  act  as  a  binder  to  cement  the  particles  to- 
gether into  large  masses.  The  hydrogen  in  combination  with 
oxygen  makes  up  the  moisture  content  of  the  fuel  and  has  little 
or  no  effect  upon  its  characteristics. 

Oxygen. — Oxygen  which  forms  20.91%  of  air  by  volume, 
and  23.15%  by  weight,  is  always  available  in  unlimited 
quantities  to  support  combustion.  When  present  in  the  fuel 
itself,  it  serves  no  useful  purpose  as  it  has  already  combined 
with  either  carbon  or  hydrogen  and  is  therefore  not  active  in 
the  combustion  process. 

Nitrogen. — Nitrogen  is  an  inert  element  brought  into  the 
combustion  process  by  the  oxygen  with  which  it  is  mixed  in 
the  air.  It  acts  as  a  diluent,  reducing  the  activity  of  the 
oxygen  and  greatly  affecting  the  temperature  of  combustion. 
The  heating  of  the  large  amount  of  nitrogen  contained  in  the 
air  used  for  combustion  is  the  chief  cause  of  heat  loss  in  the 
burning  of  fuel,  since  it  escapes  at  very  much  higher  tempera- 
tures than  it  enters  the  furnace. 

Sulphur. — Sulphur  is  found  in  all  solid  fuels  and  is  responsi- 
ble for  much  of  the  trouble  encountered  in  furnaces.  When 
present  in  small  amounts  its  effect  is  negligible  but  fuels  con- 
taining four  or  five  per  cent  sulphur  are  often  difficult  to  burn. 
While  the  sulphur  itself  is  a  combustible  element  and  unites 
readily  with  oxygen,  it  has  the  effect  of  lowering  the  fusing 
temperature  of  ash  and  causing  the  formation  of  clinkers.  This 
is  especially  true  when  it  is  present  in  the  form  of  iron  pryities. 
When  present  in  the  form  of  sulphate  of  lime,  it  has  no  heating 
value. 

Air. — Pure  dry  air  contains 20. 91  %oxy gen  and  79.09%  nitro- 
gen by  volume  and  23.15%  oxygen  and  76.85%  nitrogen  by 
weight.  The  air  supplied  for  combustion  contains  in  addition  a 
very  small  percentage  of  C02  and  a  quantity  of  water  which 
depends  upon  the  relative  humidity  and  the  temperature.  Their 
effect  on  the  combustion  process  is  so  slight,  however,  that  both 
are  usually  disregarded  in  all  except  the  most  refined 
computations. 


PRINCIPLES  OF  COMBUSTION  3 

Air  supplied  for  Combustion.— When  a  given  weight  of  fuel 
is  burned,  a  definite  amount  of  heat  is  available  for  such  work 
as  generating  steam.  The  heat  evolved  raises  the  temperature 
of  the  resultant  gases  and  this  rise  in  temperature  will  depend 
upon  the  weight  of  gas  to  be  heated  and  its  specific  heat.  It 
is  apparent  that  if  a  large  excess  of  air  be  supplied,  the  tem- 
perature will  be  lower  than  if  only  the  theoretical  amount 
were  used,  and  since  the  escaping  gas  temperature  is  always 
above  that  of  the  air  originally  supplied,  the  loss  increases 
as  the  amount  of  excess  air  is  increased.  The  weight  of  air 
supplied  for  combustion,  to  a  large  extent,  determines  the 
efficiency  of  the  process,  and  is  the  most  important  factor  in 
the  efficient  operation  of  furnaces. 

The  weight  of  air  supplied  may  be  determined  from  an 
analysis  of  the  furnace  gases  and  since  the  analysis  is  most 
conveniently  made  by  volume,  the  formulae  by  which  the 
weight  may  be  determined  directly  from  the  analysis  by  volume 
of  the  gases,  are  most  convenient.  When  pure  carbon  is  burned 
in  dry  air  the  products  of  combustion  are  carbon  dioxide, 
carbon  monoxide,  oxygen  and  nitrogen.  The  sum  of  the  carbon 
dioxide,  carbon  monoxide,  oxygen  and  nitrogen  represents  the 
entire  weight  of  gaseous  products  and  the  dry  gas  per  pound 
of  carbon  may  be  expressed  as  the  relation  of  the  total  dry 
products  of  combustion  to  the  total  weight  of  carbon  in  the 
flue  gases.  This  carbon  may  be  present  both  as  C02  and  CO. 
Carbon  represents  3/11  of  the  total  weight  of  C02  and  3/7 
of  the  total  weight  of  CO.  This  relation  may,  therefore,  be 
expressed : 

Dry  gas  per  Ib.  of  carbon  = 

100  C02+CO+02+N2 

AC02 +f  CO  AC02 +f  CO 

In  order  to  reduce  this  expression  to  a  form  in  which  per- 
centages by  volume  may  be  used,  each  term  must  be  multiplied 
by  its  relative  density  (see  table  1).  The  equation  therefore 
becomes : 

lu     ,       ,          11C02+802+7[CO+N] 
Dry  gas  per  Ib.  of  carbon  = 3(CQ2+^Q)      J- 


MECHANICAL  STOKERS 


TABLE  No.  1 
AIR  REQUIRED  FOR  COMBUSTION 


Weight 

Heat 

Pounds 

Pounds 

Total 

Liberated 

Fuel 

Reaction 

.Oper 
Pound, 

Air 
per 

Gaseous 
Products 

Relative 
Density 

B.T.U. 

per  Pound, 

Fuel 

Pound, 

per  Pound, 

Fuel 

Fuel 

Fuel 

C 

C+2O  =CO2 

2.67 

11.52 

12.52 

22 

14,540 

C 

C+0     =CO 

1.34 

5.76 

6.76 

14 

4,380 

CO 

CO+0    =C02 

0.57 

2.47 

3.47 

22 

10,150 

H 

2H+O    =H2O 

8.0 

34.56 

35.56 

9 

62,000 

s 

S+20  =S02 

1.0 

4.32 

5.32 

32 

4,050 

This  equation  is  correct  for  the  burning  of  pure  carbon  in 
dry  air  but  in  actual  practice  several  other  factors  must  be 
considered.  The  sulphur  content  of  fuel  introduces  a  slight 
error  in  the  above  equation  and  a  correction  should  be  made 
for  fuels  high  in  sulphur.  The  C02  resulting  from  the  burning 
of  one  pound  of  carbon  is  3.67  Ibs.  and  the  weight  of  S02  from 
one  Ib.  of  sulphur  is  2  Ibs.  1  Ib.  of  carbon  in  the  fuel  there- 
fore accounts  for  the  same  weight  of  products  of  combustion 
as  would  result  from  the  burning  of  1.83  Ibs.  of  sulphur.  In 
using  the  above  formula,  the  sulphur  content  can  be  corrected 
for  by  adding  to  the  total  weight  of  carbon  burned,  the  total 
weight  of  sulphur  divided  by  1.83  and  the  equation  for  determin- 
ing the  total  weight  of  the  dry  products  of  combustion  will  be 

Formula  No.  1. 


where  C  and  S  are  the  percentages  by  weight  of  carbon  and 
sulphur  contained  in  the  fuel. 

One  Ib.  of  hydrogen  unites  with  8  Ibs.  of  oxygen  which  is 
provided  by  supplying  34.56  Ibs.  of  dry  air,  the  products  of 


PRINCIPLES  OF  COMBUSTION  5 

combustion  being  9  Ibs.  of  superheated  steam  and  26.56  Ibs. 
of  nitrogen  and  the  total  weight  will  be 

(35. 56  X  per  cent  hydrogen  in  fuel) 
100 

The  nitrogen  appears  in  the  flue  gas  analysis  but  the  super- 
heated steam  is  condensed  in  the  gas  analysis  apparatus.  The 
oxygen  required  to  burn  hydrogen  therefore  does  not  appear 
in  the  analysis  while  the  nitrogen  is  present  and  the  result 
is  an  increase  in  the  percentage  of  nitrogen  and  a  decrease  in 
the  sum  of  the  percentages  of  carbon  dioxide,  carbon  monoxide 
and  oxygen.  Fuels  containing  hydrogen  when  burned  with 
the  theoretical  weight  of  air  will  give  a  smaller  percentage  of 
C02  in  the  gas  than  those  containing  no  hydrogen.  Pure  carbon 
burned  with  the  theoretical  air  supply  would  give  products  of 
combustion  containing  20.91%  carbon  dioxide.  With  natural 
gas  the  percentage  of  C02  in  the  flue  gas  when  the  theoretical 
amount  of  air  is  supplied  may  be  as  low  as  11.5%.  It  is  there- 
fore apparent  that  the  percentage  of  excess  air  cannot  be 
determined  from  the  gas  analysis  without  an  analysis  of  the 
fuel  burned.  When  a  number  of  calculations  of  excess  air 
from  gas  analyses  are  to  be  made  it  is  most  convenient  to 
prepare  a  table  or  curve  showing  the  relation  between  C02 
and  excess  air  for  a  fuel  of  the  given  analysis. 

The  following  calculations  will  show  the  method  of  pre- 
paring such  a  table: 
Assume  the  following  coal  analysis : 

Carbon 72. 60 

Hydrogen 4 . 50 

Oyxgen 4 . 25 

Nitrogen 1 . 75 

Sulphur 1 . 50 

Ash..  15.40 


100.00 


For  perfect  combustion  the  air  required  and  the  resultant 
products  of  combustion  will  be : 


6 


MECHANICAL  STOKERS 


Required  for 

Combustion  per 

Products  of  Combustion, 

Weight  per 
Pound  of  Coal, 

Pound  of  Coal, 
Pounds 

Pounds  per  Pound  of  Coal 

Pounds 

02 

Air 

C02 

02 

N2 

H20 

S02 

c 

.7260 

1.936 

8.364 

2.662 

6.428 

H2 

.0450 

.360 

1.555 

.... 

.... 

1.195 

.405 

O2 

0425 

.043 

N2 

0175 

.018 

S 

.0150 

.015 

.065 

.... 

.... 

.050 

.030 

Ash 

.1540 

1.0000 

2.311 

9.984 

2.662 

.043 

7.691 

.405 

.030 

A  correction  must  be  made  in  the  oxygen  and  air  required 
for  combustion,  due  to  the  presence  of  oxygen  in  the  original 
fuel.  This  also  affects  the  weight  of  nitrogen  as  calculated 
above.  A  second  correction  is  necessary  because  the  sulphur 
dioxide  is  absorbed  in  the  gas  analysis  apparatus  along  with 
the  carbon  dioxide.  The  corrected  quantities  will  be: 


Required  for 
Combustion  per 
Pound  of  Coal, 

Products  of  Combustion, 
Pounds  per  Pound  of  Coal 

02 

Air 

CO2 

02 

N2 

H20 

SO2 

Correction  for  O2  in 
coal 

2.311 
-.043 

9.984 
-.186 

2.662 

.043 
-.043 

7.691 
-.143 

.405 

.030 

Correction  for  SO2 
absorbed  as  CO2 

2.268 

9.798 

2.662 
+  .030 

.000 

7.548 

.405 

.030 
-.030 

2.268 

9.798 

2.692 

.000 

7.548 

.405 

.000 

The  weight  of  air  required  is  9.798  Ibs.  per  Ib.  of  coal  and 
the  products  of  combustion  will  weigh  10.645  Ibs.     If  10% 


PRINCIPLES  OF  COMBUSTION  7 

excess  air  be  supplied  there  will  be  an  increase  of  .98  Ib.  in 
the  products  of  combustion,  .754  Ib.  of  which  will  be  nitrogen 
and  .226  Ib.  oxygen.  The  weight  in  Ibs.  of  the  products  of 
combustion  for  varying  amounts  of  excess  air  will  be : 

WEIGHT   OF   PRODUCTS   OP   COMBUSTION.     POUNDS   PER   POUND   OP  COAL 


Per  Cent  Excess  Air 

0 

10 

20 

30 

40 

50 

60 

80 

100 

CO2 
02 
N2 
H20 

Total  

2.692 
.000 

7.548 
.405 

2.692 
.226 
8.302 
.405 

2.692 
.452 
9.056 
.405 

2.692 
.678 
9.810 
.405 

2.692 
.904 
10.564 
.405 

2.692 
1.130 
11.318 
.405 

2.692 
1.356 
12.072 
.405 

2.692 
1.808 
13  .  580 
.405 

2.692 
2.260 
15.088 
.405 

10.645 

11.625 

12.605 

13.585 

14.565 

15.545 

16.525 

18.485 

20.445 

Total  dry 
products 

10.240 

11.220 

12.200 

13.180 

14.160 

15.140 

16.120 

18.080 

20.040 

The  weight  of  C02,  02  and  N2  expressed  as  percentages  oi 
the  total  dry  products  will  be: 


Per  Cent  Excess  Air 


0 

10 

20 

30 

40 

50 

60 

80 

100 

CO2 

26.29 

23.99 

22.05 

20.42 

19.01 

17.78 

16.70 

14.88 

13.43 

02 

.00 

2.01 

3.71 

5.14 

6.38 

7.46 

8.41 

10.00 

11.28 

N2 

73.71 

74.00 

74.24 

74.44 

74.61 

74.76 

74.89 

75.12 

75.29 

Gras  analyses  are  usually  given  in  terms  of  volume  and  it 
is  therefore  preferable  to  convert  the  above  results  into  per- 
centages by  volume.  This  is  done  by  dividing  the  percentage 
by  weight  of  each  constituent  by  its  relative  density  and  then 
dividing  each  value  thus  obtained  by  the  sum  of  the  values. 


8  MECHANICAL  STOKERS 

PER  CENT  VOLUME  OF  DRY  PRODUCTS 


Per  Cent  Excess  Air 

0 

10 

20 

30 

40 

50 

60 

80 

100 

C02 

18.50 

16.77 

15.34 

14.14 

13.  n 

12.22 

11.44 

10.14 

9.13 

0, 

.00 

1.93 

3.54 

4.88 

6.05 

7.04 

7.93 

9.38 

10.53 

N2 

81.50 

81.30 

81.12 

80.98 

80.84 

80.74 

80.63 

80.48 

80.34 

In  the  use  of  gas  analysis  apparatus,  particular  care  should 
be  taken  to  secure  an  average  sample.  In  boiler  furnaces  it 
is  difficult  if  not  impossible  to  secure  an  average  sample  before 
the  gas  enters  the  heating  surface  of  the  boiler,  because  the 
gases  rising  from  different  parts  of  the  fuel  bed  are  not  of 
uniform  analysis.  The  mixing  effect  of  contact  with  the  tubes 
and  changes  in  direction  of  gas  travel  caused  by  the  boiler 
baffles  is  such  that  an  average  sample  can  be  secured  in  the 
last  pass  of  the  boiler  but  samples  taken  at  points  nearer  the 
furnace  are  always  of  questionable  value. 

Temperature  of  Combustion. — The  function  of  a  boiler 
furnace  is  to  heat  gas.  The  boiler  then  cools  this  gas,  trans- 
mitting the  heat  to  the  steam  and  water.  Since  the  extent  to 
which  the  cooling  process  can  be  carried  is  limited  on  account 
of  the  temperature  of  the  water  in  the  boiler,  it  follows  that 
for  best  results,  the  gas  should  leave  the  furnace  as  hot  as 
possible  and  every  factor  affecting  the  temperature  attain- 
able should  be  carefully  considered. 

The  temperature  of  the  products  of  combustion  depends 
upon  the  heat  units  produced,  the  weight  of  the  products  of 
combustion  and  the  mean  specific  heat  between  the  initial  and 
final  temperatures  of  the  air  and  fuel.  Since  the  specific  heat 
will  vary  over  a  range  of  temperature  it  is  impossible  to  make 
a  direct  calculation  of  combustion  temperatures.  A  value  must 
be  assumed  for  the  mean  specific  heat  based  on  an  assumed 
temperature  of  combustion  and  a  trial  calculation  made.  If 
this  calculation  shows  a  temperature  different  from  the  assumed 
temperature,  a  new  value  for  the  specific  heat  must  be  taken, 
assuming  a  maximum  temperature  between  the  first  assumed 


PRINCIPLES  OF  COMBUSTION  9 

value  and  the  calculated  value.  The  second  calculation  will 
be  more  accurate  than  the  first  and  this  process  may  be 
repeated  until  the  desired  degree  of  accuracy  is  secured. 

In  actual  practice,  three  factors  affect  the  maximum 
temperature  as  follows :  first,  it  is  impossible  to  secure  complete 
combustion  without  an  excess  over  the  theoretical  amount  of 
air  required,  and  the  temperature  decreases  with  the  increase 
in  the  amount  of  excess  air  supplied;  second,  an  excess  of  air 
being  required  for  complete  combustion,  it  is  evident  that  if 
this  excess  be  reduced  too  far,  incomplete  combustion  results, 
and  the  full  amount  of  heat  in  the  fuel  is  not  liberated;  third, 
radiation  from  the  burning  fuel  carries  away  heat.  In  the  case 
of  a  boiler  furnace  where  a  part  of  the  heating  surface  is  in 
proximity  to  the  fuel  bed,  a  large  amount  of  heat  is  absorbed 
by  direct  radiation  and  the  temperature  is  thereby  reduced. 

Since  the  quantity  of  heat  radiated  from  the  burning  fuel 
is  a  function  of  the  time  as  well  as  the  temperature,  it  follows 
that  the  lower  the  rate  of  combustion,  the  greater  is  the  per- 
centage of  heat  carried  away  by  radiation.  The  rate  of  com- 
bustion, therefore,  affects  the  temperature  of  the  fire,  the 
temperature  increasing  as  the  combustion  rate  increases,  pro- 
vided of  course,  the  relation  of  fuel  to  air  is  maintained  con- 
stant. 

Temperature  of  the  Furnace.— The  temperature  of  a  boiler 
furnace  is  affected  by  a  number  of  factors  and  varies  in  dif- 
ferent parts  of  the  furnace.  The  burning  fuel  is  at  one  tem- 
perature, but  this  temperature  varies  for  different  stages  of 
combustion,  the  gases  leaving  the  fuel  bed  are  at  a  different 
temperature,  and  the  furnace  brick  work  at  a  still  different 
temperature.  The  heating  surface  of  the  boiler  which  is  in 
proximity  to  the  furnace  being  only  a  few  degrees  hotter  than 
the  water  in  the  boiler,  has  a  decided  cooling  effect  upon  the 
furnace  and  prevents  the  other  parts  of  the  furnace  from  reach- 
ing the  temperatures  that  would  otherwise  result.  It  is  ap- 
parent that  the  temperature  may  be  increased  by  moving  the 
boiler  surface  farther  from  the  furnace  and  interposing  a  roof 
of  refractory  material.  The  design  of  the  furnace,  therefore, 
has  an  important  effect  on  the  temperature  and  no  accurate 
knowledge  of  the  amount  of  excess  air  present  can  be  secured 


10  MECHANICAL  STOKERS 

from  furnace  temperature  alone.  Temperatures  of  over  3000°  P. 
may  be  secured  in  furnaces  burning  coal  at  high  rates  of  combus- 
tion and  admitting  not  more  than  30%  excess  air,  but  such  tem- 
peratures are  seldom  attained  in  actual  practice  except  for  short 
intervals,  and  rapid  burning  of  furnace  brick  work  would 
result  if  such  temperatures  were  maintained  for  long  periods 
of  time.  In  actual  practice,  the  temperatures  vary  from  2400° 
to  3000°  when  furnaces  are  operated  economically  and  are 
carrying  fair  overloads,  and  will  go  as  low  as  1500°  F,  at 
low  ratings,  and  with  inefficient  operation. 

Efficiency  of  Combustion. — While  the  burning  of  fuel  in  a 
furnace  and  the  utilization  of  the  heat  thus  liberated  for  the 
generation  of  steam,  are  two  separate  and  distinct  processes, 
they  are  so  closely  related,  that  the  factors  affecting  efficiency 
are  most  conveniently  considered  for  the  furnace  and  boiler 
taken  as  a  unit. 

The  complete  list  of  losses  is  as  follows: 

1.  Heat  carried  away  in  the  dry  chimney  gases. 

2.  Unburned  gases  discharged  with  the  products  of  combustion. 

3.  Superheated  steam  formed  by  burning  hydrogen  in  the  fuel  and  by 
evaporating  the  moisture  in  the  coal. 

4.  Superheating  the  moisture  in  the  air. 

5.  Combustible  discharged  to  the  ash  pit. 

6.  Sensible  heat  in  the  ash  pit  refuse. 

7.  Soot  and  cinders  deposited  in  the  gas  passages  and  carried  away 
with  the  chimney  gases. 

8.  Radiation. 

Fine  particles  of  fuel  which  sift  through  the  grates  are 
sometimes  improperly  included  with  the  losses.  This  is  a 
deliberate  waste  and  has  no  place  in  the  list  of  losses,  because 
the  sif tings  are  easily  reclaimed  and  should  be  returned  to  the 
furnace. 

Heat  Carried  Away  by  the  Dry  Chimney  Gases.— The  fuel 
and  air  used  for  combustion  are  supplied  at  atmospheric  tem- 
perature and  the  heat  necessary  to  raise  this  temperature  to 
that  of  the  escaping  gases  is  lost,  the  amount  of  the  loss  depend- 
ing upon  the  rise  of  temperature  and  the  weight  of  gas. 

The  weight  of  gas  is  calculated  from  formula  No.  1,  (Page 
4)  when  the  weight  of  C  burned  per  pound  of  coal  as  fired  is 
known.  This  can  be  determined  from  an  analysis  of  the  fuel 


PRINCIPLES  OF  COMBUSTION  11 

and  ash  with  an  allowance  for  the  weight  of  carbon  carried 
away  by  the  escaping  gases  or  deposited  in  gas  passages. 

If  C6  =  %  C   burned  per  Ib.  of  coal  as  fired,    corrected  for 

sulphur    content;     C6  =  %  C    in    actual    Coal+(]~g3)  ~K%  c 

carried  away  by  gases  +%  C  rejected  to  the  ash  pit].  (The  %  C 
lost  in  the  ash  pit  being  determined  by  subtracting  the  %  ash 
in  the  coal  from  the  %  of  dry  refuse  in  the  pit.) 

Let  Tf=  temperature  of  the  escaping  gases  in  degrees  F., 

Ta=  temperature  of  the  air  in  degrees  F. 
Specific  heat  of  dry  gas  =  .24. 
Then  heat  carried  away  by  dry  chimney  gases  per  Ib.  of 

(CO+N) 


3  (C0+C02) 

If  great  accuracy  is  desired  a  calculation  of  the  average 
specific  heat  between  Tf  and  Ta  should  be  made  and  the  actual 
value  used  instead  of  .24.  For  ordinary  calculations,  however, 
.24  is  sufficiently  accurate. 

Loss  due  to  Unburned  Gases  Discharged  with  the  Products 
of  Combustion.  —  In  determining  the  loss  due  to  incomplete 
combustion,  it  is  assumed  that  the  hydrogen  has  all  been  burned 
and  that  the  incomplete  combustion  is  represented  entirely 
by  CO  in  the  escaping  gases.  The  equation  for  determining 
this  loss  may  be  derived  as  follows  : 

The  loss  due  to  incomplete  combustion  is  determined  as 
follows  : 

CinCO  =  f  CO; 


C  in  gas  =  f  CO+A  C02; 

3  QO 

Q       *.    r-F^r  =  proportional  part  of  C  remaining  in  the  form 

fco+ACO2   ofco 

Multiplying  each  member  by  its  relative  density,  this  expres- 
sion reduces  to  (pQ.pQ  )  =  pounds  of  C  in  CO  per  pound  of  C 
burned. 


12  MECHANICAL  STOKERS 

The  combustion  of  1  pound  of  C  contained  in  CO  to  C02 
generates  10,150  B.T.U. 

The  loss  due  to  CO  per  pound  C=  (CQ^O  )  X 10, 150. 
Loss  per  pound  of  coal  as  fired    =  (QQ  .  QQ  )  *  10, 150  X  C&. 

This  expression  is  correct  if  the  only  unburned  fuel  in  the 
furnace  gases  is  present  in  the  form  of  CO.  It  is  known,  how- 
ever, that  this  is  not  correct  although  the  extent  and  nature 
of  the  unconsumed  hydro-carbons  is  not  fully  understood. 

When  coal  is  heated  to  a  temperature  of  500°  F.,  distillation 
of  the  volatile  content  begins  and  both  gaseous  and  liquid 
hydro-carbons  are  liberated.  The  rate  of  liberation  increases 
until  the  temperature  reaches  about  1000°  F.  after  which  there 
is  a  gradual  decrease. 

These  hydro-carbons  are  very  complex  in  their  structure 
and  no  accurate  information  is  available  as  to  the  exact  steps 
in  their  complete  combustion.  It  is  probable,  however,  that 
the  hydro-carbons  are  distilled  from  the  coal  in  the  form  of 
liquid  tar  and  tar  vapors  which  undergo  chemical  changes 
when  brought  into  the  region  of  high  temperature  with  a  cer- 
tain amount  of  free  oxygen  present,  and  there  is  every  indica- 
tion that  some  of  the  combustible  gases  evolved  are  discharged 
unburned  when  an  attempt  is  made  to  reduce  the  excess  air 
too  far  or  where  the  furnace  is  not  of  sufficient  size  to  allow 
time  for  the  completion  of  the  combustion  process.  A  careful 
analysis  of  boiler  tests  which  have  been  conducted  with  the  ut- 
most care  shows  that  the  unaccounted  for  loss  is  greater  in  those 
cases  where  the  furnace  is  small  than  where  furnaces  of  liberal 
proportions  are  used.  Since  the  unaccounted  for  loss  could 
not  be  accounted  for  in  any  other  way,  it  is  apparent  that  in 
the  smaller  furnaces  there  is  a  certain  loss  due  to  incomplete 
combustion  of  the  hydro-carbons,  which  is  not  detected  by 
the  gas  analysis  and  which  is  not  necessarily  accompanied  by 
the  production  of  smoke.  A  similar  condition  exists  when  com- 
paring tests  run  at  different  ratings  on  the  same  furnace.  At 
the  higher  ratings  the  unaccounted  for  loss  increases  in  spite 
of  the  fact  that  the  radiation  which  is  an  important  factor 
in  the  unaccounted  for  loss  is  a  smaller  percentage  than  at  the 


PRINCIPLES  OF  COMBUSTION  13 

lower  ratings.  The  increase  in  unaccounted  for  loss  is  there- 
fore evidently  due  to  some  form  of  incomplete  combustion 
which  is  not  detected  in  the  ordinary  gas  analysis  instrument. 

Loss  Due  to  Superheated  Steam  Formed  by  Burning  Hydro- 
gen in  the  Fuel  and  by  Evaporating  the  Moisture.  —  For  each 
unit  weight  of  hydrogen  in  the  fuel,  nine  times  this  weight 
of  superheated  steam  results  from  burning  the  hydrogen. 

The  moisture  in  the  fuel  is  also  evaporated  and  escapes  as 
superheated  steam.  The  resultant  loss  may  be  determined  as 
follows  : 


=  %  moisture  in  coal  as  fired; 
H  =  %  H  in  coal  as  fired; 
.50  in  specific  heat  of  superheated  steam. 


Loss  due  to  H2O  in  coal  =  ^?[  (212  -Ta)  +970.4  +.50  (3*/-212)j 
Loss  due  to  H  in  coal     =^j!T(212-T0)+970.4+.50  (^-212)1 

These  losses  may  be  expressed  in  one  equation: 


.50  (7>-212)   . 


While  hydrogen  in  fuel  adds  materially  to  its  heating  value, 
it  also  increases  the  necessary  loss  due  to  the  escape  of  super- 
heated steam  and  thereby  lowers  the  efficiency.  Other  factors 
being  equal,  a  fuel  with  low  hydrogen  content  would  be  desir- 
able from  the  standpoint  of  most  efficient  combustion. 

In  spite  of  the  fact  that  the  moisture  content  is  also  responsi- 
ble for  a  loss  of  heat,  many  fuels  can  be  more  satisfactorily 
burned  by  the  addition  of  from  3%  to  5%  surface  moisture. 
This  is  not  due  to  any  chemical  action  of  the  moisture  but  to 
its  effect  on  the  nature  of  the  fuel  bed.  This  matter  will  be 
referred  to  in  detail  later. 

Combustible  Discharged  at  the  Ash  Pit.  —  The  heat  loss  due 
to  the  presence  of  combustible  in  the  ash  pit  is  usually  over 
estimated.  A  comparatively  small  percentage  of  unburned 
carbon  is  plainly  visible  and  may  appear  to  represent  a  much 


14  MECHANICAL  STOKERS 

larger  loss  than  actually  exists.     The  loss  per  pound  of  coal 
as  fired  may  be  calculated  from  the  following  formula : 

,  _  „  (%  refuseX%  Gin  refuse) 

100  ~~' 

in  which  H  =  calorific  value  of  carbon  and  h  =  heat  loss  per 
pound  of  coal  as  fired. 

Sensible  Heat  in  the  Ash  Pit  Refuse.— The  hot  ash  and 
refuse  carry  a  certain  amount  of  sensible  heat  away  from 
the  furnace.  The  specific  heat  of  the  refuse  is  about  .28  and 
the  loss  from  this  source  would  be  represented  by  the  follow- 
ing equation: 

T0X.28X%  refuse  per  Ib.  of  coal 
100 

The  temperature  at  which  the  refuse  is  discharged  varies 
considerably  with  different  methods  of  firing  but  in  most 
cases,  averages  from  1500°  to  2000°  F. 

Soot  and  Cinders  Deposited  in  the  Gas  Passages  or  Carried 
Away  by  the  Chimney  Gases. — In  a  well  designed  and  properly 
operated  furnace,  the  fuel  loss  due  to  soot  accumulations  is 
negligible. 

With  poor  design  and  improper  operation,  dense  clouds  of 
black  smoke  may  be  discharged  and  a  small  amount  of  carbon 
in  the  form  of  soot  is  thus  lost  but  it  is  doubtful  if  this  loss 
ever  reaches  1%  of  the  fuel.  The  large  loss  of  which  dense 
smoke  is  the  visible  sign,  is  not  due  to  the  particles  of  soot 
discharged  but  to  the  incomplete  combustion  of  the  hydro- 
carbons. 

It  is  well  known  that  a  rapidly  moving  current  of  air  or 
gas  will  carry  in  suspension  particles  heavier  than  air,  espe- 
cially if  they  are  small.  In  this  manner  cinders  are  carried 
over  into  the  boiler  passes  and  breeching  and  are  deposited  at 
points  where  there  is  a  sudden  change  in  the  direction  of  flow 
or  a  reduction  in  velocity  due  to  increased  crossed  section  of 
the  gas  passage.  .  The  lighter  particles  being  held  in  suspen- 
sion the  longest,  are  discharged  to  the  atmosphere  with  the 
chimney  gases. 


PRINCIPLES  OF  COMBUSTION  15 

The  loss  due  to  cinders  may  be  as  high  as  2%  of  the  total 
coal  fired,  especially  when  furnaces  are  operated  at  high 
capacities  and  gases  are  traveling  at  high  velocity. 

Radiation  and  Miscellaneous. — The  radiation  from  furnace 
and  boiler  brickwork  depends  very  largely  upon  the  construc- 
tion of  the  furnace  walls.  At  one  time  it  was  believed  that  this 
loss  could  be  materially  reduced  by  leaving  an  air  space  in 
the  center  of  the  walls  but  it  has  been  found  by  careful  experi- 
ments that  the  radiation  across  this  air  space  actually  increased 
the  loss,  and  that  filling  the  air  spaces  with  sand  or  cinders 
resulted  in  a  substantial  decrease. 

The  temperature  of  a  furnace  wall  will  not  increase  in 
direct  proportion  to  the  rate  of  burning  fuel  and  since  the 
area  of  radiating  surface  is  constant,  the  per  cent  of  the  total 
heat  which  is  lost  by  radiation  thereby  decreases  as  the  rating 
is  increased. 

When  a  boiler  is  banked  the  amount  of  fuel  required  to 
keep  it  in  a  standard  condition  is  sometimes  referred  to  as 
radiation  loss,  but  this  is  not  correct  for  the  reason  that  under 
such  conditions  the  fuel  required  to  maintain  the  banked  con- 
dition is  burned  very  inefficiently  and  only  a  small  part  of  the 
heat  apparently  generated  is  actually  available. 

The  leakage  of  cold  air  through  cracks  in  brickwork  and 
through  the  brick  itself  is  often  a  source  of  considerable  loss, 
especially  where  the  brickwork  is  unprotected  and  the  boilers 
are  operated  with  a  high  draft  throughout  the  setting. 


CHAPTER  II 
MECHANICAL  STOKERS 

From  the  time  that  coal  was  first  fired  on  a  grate,  there 
seemed  to  be  a  pretty  good  understanding  that  the  factors 
effecting  the  proper  burning  of  coal  were : 

(a)  A  continuous  feed  of  coal. 

(b)  Proper  proportion  of  air  and  a  mixture  of  air  and  gases. 

(c)  High  temperatures. 

It  was,  therefore,  the  ambition  of  most  inventors  in  the 
early  history  of  coal-burning  devices,  to  conceive  of  some 
combination  of  things  that  would  provide  these  conditions  and 
thus  lead  to  the  burning  of  coal  without  smoke. 

D.  K.  Clark,  in  his  excellent  treatise  "The  Steam  Engine," 
gives  some  very  interesting  history  of  smoke  contrivances  that 
came  from  ambitious  inventors  in  the  early  days — a  review  of 
some  of  the  most  important  ones  is  interesting,  as  leading  up 
to  the  mechanical  feeding  of  fuel  to  furnaces, 

SMOKE-PREVENTION   CONTRIVANCES 

Watt — 1785  (English}. — James  Watt,  the  inventor  of  the 
steam  engine,  conceived  the  idea  that  the  volatile  gases  of  coal 
should  be  distilled  slowly,  and  in  one  of  his  first  furnaces  (Fig. 
1),  the  coal  was  piled  up  at  the  front  of  the  grate  and  then 
gradually  pushed  farther  towards  the  rear,  giving  the  volatile 
gases  a  chance  to  distill  slowly.  The  coal  piled  up  in  front 
served  to  shut  off  excess  air  admittance  to  the  furnace. 

Robertson — 1800  (English}. — J.  and  J.  Robertson  thought  it 
was  necessary  to  admit  air  over  the  fire  in  order  to  completely 
burn  the  volatile  gases.  Their  invention  covered  the  admission 
of  a  sheet  of  air  immediately  over  the  coking  part  of  the  fuel 

16 


MECHANICAL  STOKERS 


17 


bed  (Fig.  2).  It  is  interesting  to  note  that  this  patent  involves 
a  coal  hopper,  a  fuel-burning  structure  and  a  refuse  disposal 
space;  these  are  fundamental  ideas  and  were  later  worked  out 


FIG.  1. — Watt's  Smokeless  Furnace. 

and  developed  in  connection  with  the  design  of  mechanical 
stokers. 

Wakefield—1810  (English).— John  Wakefield  had  the  same 
general  idea  that  air  was  required  over  the  fuel  bed  for  smoke 
prevention  and  worked  out  the  idea  of  passing  air  through  a 


FIG.  2. — Robertson's  System, 

hollow  bridgewall,  thus  heating  the  air  before  admitting  it  (Fig. 
3).  This  appears  to  be  the  first  idea  of  admitting  air  over  the 
fire  at  a  point  near  the  bridgewall. 

Gregson — 1816   (English) — Gregson,  in  1816,  used  piers  or 
bridges  in  a  furnace,  in  order  to  obtain  a  mixture  of  the  gases 


18 


MECHANICAL  STOKERS 


(Fig.  4).  By  the  installation  of  two  bridgewalls,  the  gases  from 
the  fuel  bed  were  made  to  separate,  part  passing  over  the  bridge 
and  part  underneath,  and  then  coming  in  contact  again  with 


FIG,  3,— Wakefield's  System. 

an  air  admission  before  passing  underneath  the  second  bridge- 
wall. 

This  is  probably  the  first  idea  of  using  bridges  or  piers  for 
mixing  and  intermingling  gases,  and  was  used  quite  success- 


FIG.  4. — Gregson's  System. 

fully  in  later  years  in  hand-fired  furnaces  for  smoke  prevention. 

Witty— 1828  (English).— R.  Witty,  it  appears,  had  the  first 

idea  of  using  the  gas-producer  effect  in  burning  coal.    Coal  was 


MECHANICAL  STOKERS 


19 


admitted  slowly,  and  a  deep  well  provided  in  which,  the  fuel 
was  burned  (Fig.  5).  The  volatile  gases  were  made  to  pass 
upward  over  an  incandescent  fuel  bed  and,  in  this  way,  main- 
tained high  temperatures.  It  is  said  that  this  system  failed  on 
account  of  lack  of  air  supply  for  the  volatile  gases. 

Shanter — 1834  (English}. — John  Shanter,  following  a  similar 
idea  of  Gregson,  used  an  inverted  arch  back  of  the  bridgewall, 
and  in  combination  with  this,  admitted  air  back  of  the  bridge- 
wall  endeavoring  to  obtain  a  proper  mixture  of  air  and  gases. 


FIG.  5. — Witty's  System  of  Furnace. 

Gray  and  Shanter — 1835  (English). — Gray  &  Shanter 
patented  a  system  of  two  grates,  one  inclined  for  coking  the 
coal,  and  the  other  for  burning  the  fixed  carbon.  The  coal,  when 
the  gases  were  distilled,  was  pushed  back  on  the  second  grate. 
This  is  probably  the  fundamental  idea  of  a  down  draft  furnace 
that  was  used  quite  successfully  in  later  years. 

Rodda — 1838  (English). — R.  Rodda  followed  the  same  idea 
as  Gregson,  using  an  inverted  arch  in  the  center  of  the  grate. 
In  order  to  obtain  a  proper  mixture,  the  air  and  gases  were 
forced  underneath  this  bridge. 


20 


MECHANICAL  STOKERS 


Williams— 1839  (English}.— C.  "Wye  Williams  used  an 
Argand  blower  to  obtain  a  mixture  of  air  and  gases  (Fig.  6). 
Air  was  introduced  behind  the  bridgewall  in  finely  divided 
streams,  either  by  driving  air  through  perforated  pipes,  or 
through  perforated  plates.  In  some  of  his  patents,  an  ingenious 
idea  for  admitting  air  over  the  fire  was  obtained  by  taking  out 
the  center  grate  bars  of  a  furnace  and  installing  a  perforated 
plate  so  that  air  could  be  admitted  about  3  in.  over  the  fire.  This 
was  very  effective  and  streams  of  fire  seemed  to  come  from  the 
orifices  of  this  plate. 


FIG.  6. — Williams'  System. 

Prideaux — 1853  (English}. — T.  S.  Prideaux  is  probably  the 
first  one  who  conceived  the  idea  of  automatically  admitting  air 
through  the  fire  doors  after  each  charge  of  fuel. 

In  the  fire  front,  valves  were  used  and  controlled  by  hand 
levers,  so  that  when  opened,  a  full  charge  of  air  was  admitted, 
and,  by  means  of  water  cylinders,  the  system  of  valves  and 
levers  gradually  fell  and  shut  off  the  air. 

Langen — 1862  (English). — Langen  developed  the  idea  of  an 
inclined  stepped  grate  wherein  the  coal  was  pushed  from  one 
grate  to  another  until  completely  burned  (Fig.  7).  This  is  a 
fundamental  idea  that  probably  led  to  the  development  of  a 
stepped  grate  mechanical  stoker. 

Barber — 1881  (English). — From  the  present  knowledge  of 
the  requirements  for  burning  coal  without  smoke,  the  early 
inventors  were  working  under  a  serious  handicap  since  all  of 


MECHANICAL  STOKERS 


21 


the  contrivances  were  installed  in  connection  with  internally- 
fired  boilers. 


FIG.  7. — Langen's  Stepped  Grate. 


FIG.  8. — Barber's  Stepped  Furnace. 

Barber's  stepped  furnace  (Fig.  8),  seems  to  be  the  first 
instance  where  an  extended  firebrick  combustion  chamber  is 
used  providing  proper  distance  between  the  fuel  bed  and  the 


22  MECHANICAL  STOKERS 

cool  surfaces  of  the  boiler.  In  all  previous  contrivances  up  to 
this  time,  the  grate  was  placed  very  close  to  the  cool  surfaces 
of  the  boiler,  and  the  gases  had  very  little  time  in  which  to 
burn. 

No  doubt,  this  application  feature  was  the  reason  for  so 
many  of  the  early  contrivances  failing  insofar  as  smoke  abate- 
ment was  concerned. 

MECHANICAL  FEEDING   OF   COAL  TO  FURNACES 

The  many  attempts  to  fire  coal  by  hand,  and  to  burn  it  with- 
out producing  smoke  led  to  an  endeavor  to  mechanically  produce 
the  conditions  necessary  for  proper  combustion. 

This  development  work  seemed  to  be  along  the  lines  of  pro- 
ducing these  conditions  by  means  of  a  traveling  grate,  an  inclined 
grate  or  a  screw  conveyor.  Thus,  the  fundamental  principles  of 
the  present  forms  of  traveling  grate,  overfeed  and  underfeed 
stokers,  were  established. 

The  principle  of  the  traveling  grate  stoker  is  to  deposit  the 
fuel  from  suitable  hoppers  on  a  grate  that  moves  slowly  inward. 
The  coal  is  first  ignited  and  the  volatiles  driven  off  and  progres- 
sing farther  inward  on  the  same  grate,  the  fixed  carbon  is  con- 
sumed. Air  is  admitted  through  the  grates.  At  the  end  of  the 
grate  travel,  at  the  rear  of  the  furnace,  the  ash  is  dumped  into 
a  pit. 

In  the  overfeed  stokers,  the  coal  from  suitable  hoppers  is 
pushed  in  by  mechanical  means  onto  a  coking  plate  where  the 
volatile  gases  are  distilled.  The  grate  bars  are  inclined  and 
given  a  movement  and,  aided  by  gravity,  the  fuel  progresses 
over  the  grate  and  the  fixed  carbon  is  consumed.  Air  is  admitted 
through  the  grates.  The  ash  is  collected  at  the  bottom  of  the 
incline  and  there  dumped  into  the  ash  pit  periodically  or  crushed 
by  ash  grinders. 

In  the  underfeed  type  of  stoker,  the  coal  is  forced  by  mechani- 
cal means  into  a  magazine  or  retort  extending  into  the  furnace. 
The  coal  is  then  pushed  upwards  into  the  incandescent  fuel 
bed.  Air  under  pressure  is  forced  through  the  fuel  bed.  Ash 
is  collected  on  plates  at  the  side  of  the  retorts  or  on  dump  grates 
at  the  rear  of  the  furnace  and  periodically  dropped  into  the 
ash  pit. 


MECHANICAL  STOKERS 


23 


TRAVELING   GRATE  STOKERS 

Brunton — 1819  (English). — The  first  mechanical  stoker  was 
brought  out  in  England  in  1819  by  Wm.  Brunton.  This  was  a 
traveling  grate  stoker  consisting  of  a  circular  grate  revolving 
on  a  vertical  spindle.  The  coal  was  fed  to  the  grate  from  a 
suitable  hopper,  and  as  the  grate  gradually  revolved,  the  volatile 
gases  were  distilled  slowly,  and  finally  the  fixed  carbon  was 
consumed  and  ash  and  refuse  pulled  from  the  grate. 


FIG.  9. — Jukes'  Chain  Grate  Stoker  of  1841. 

Bodmer — 1834  (English). — The  first  traveling  grate  that 
moved  from  the  front  of  the  boiler  inward  to  the  bridgewall 
was  brought  out  by  Bodmer  in  1834.  The  grates  moved  slowly 
inward  and  were  then  dropped  and  returned  to  the  front  on 
rails  or  by  means  of  screws.  In  the  forward  movement,  the  bars 
by  setting  in  spiral  screws  were  given  a  reciprocating  move- 
ment, the  aim  being  to  break  up  the  caking  fuel  bed. 

Jukes — 1841  (English). — The  original  traveling  chain  grate 
stoker  was  brought  out  in  England  by  Jukes  in  1841  (Fig.  9). 
The  principle  of  this  stoker  was  a  number  of  longitudinal  bars 
connected  together  to  form  an  endless  chain.  This  moved  slowly 


24  MECHANICAL  STOKERS 

from  the  front  of  the  furnace  inward  towards  the  rear,  the 
coal  being  deposited  on  the  grate  from  a  suitable  hopper  and 
moving  inward  on  the  grate,  the  volatile  gases  were  distilled 
slowly.  The  ash  was  dropped  over  the  rear  of  the  grate. 

Weller — 1871  (American). — The  earliest  record  of  a  travel- 
ing grate  stoker  in  America  was  that  brought  out  by  Royal  F. 
Weller  in  1871  (Fig.  10).  In  this  grate,  the  Jukes  principle 
was  used  with  the  exception  that  the  arrangement  of  the  bars 


1  — 

1 

)4r' 

L  _ 

L.  _. 

L  —  J_ 

n  rH- 

- 

T-; 

:rf  " 

*!<U 

/ 

/ 

?| 

i 

1 

I 

L 

^jfr 

> 

9\ 

t£=i. 

r 

r'       V 

T 

- 

^ 

;  3 

-M- 

u 

FIG.  10.— Weller  Traveling  Grate  Stoker. 

were  made  transversely  instead  of  longitudinally.  Each  of  the 
bars  strung  transversely  across  the  furnace  were  fastened  to- 
gether by  links  and,  in  this  way,  an  endless  chain  was  made. 

Coxe — 1893  (American). — E.  B.  Coxe  employed  many  novel 
features  in  a  chain  grate  stoker,  the  most  important  of  which 
were  the  method  of  air  distribution  and  the  design  of  the  grates 
for  burning  small  sizes  of  anthracite  coals. 


MECHANICAL  STOKERS 


25 


OVERFEED    STOKER 

Hall — 1845  (English). — Probably  the  first  attempt  to 
mechanically  feed  a  furnace  fire  by  means  of  inclined  grate  bars 
was  brought  out  by  Hall  in  1845  (Fig.  11).  Fuel  was  supplied 
from  a  hopper,  in  back  of  which  channels  were  left  for  admitting 
air  over  the  fire.  The  fuel  was  pushed  onto  inclined  reciprocat- 
ing grate  bars.  This  original  inclined  overfeed  stoker  had  the 
same  general  design  principles  as  the  present-day  types,  namely, 
a  hopper  for  feeding  fuel,  an  inclined  grate  for  burning  the 
fuel,  and  a  sliding  shelf  for  disposing  of  ash  and  refuse. 

Vicars— 1867  (English) .—This  development  of  the  inclined 
grate  stoker  is  interesting  in  that  coal  was  fed  onto  grate  bars 


FIG.  11. — Samuel  Hall's  Smoke-preventing  Apparatus. 

by  means  of  plungers  (Fig.  12),  and  the  inclined  grate  bars 
were  given  a  reciprocating  motion  to  slowly  advance  the  fuel. 
This  stoker  was  used  extensively  in  England. 

McDougall — 1880  (English). — This  stoker  covered  the  most 
complete  combinations  of  the  elements  making  up  an  inclined 
grate  stoker.  The  coal  was  fed  from  suitable  hoppers,  being 
pushed  onto  a  coking  plate  which  projected  somewhat  into  the 
furnace.  Air  was  admitted  over  the  fire  at  this  point.  The 
stroke  of  the  rams  feeding  fuel  into  the  furnace  could  be  varied. 
The  grate  bars  were  given  a  reciprocating  movement  that 
gradually  fed  the  fuel  from  the  front  of  the  stoker  inwards. 
An  arch  was  used  for  ignition  purposes.  Ash  and  refuse  were 
collected  on  grates  at  the  bottom  of  the  furnace. 


26 


MECHANICAL  STOKERS 


Murphy — 1878  (American). — Thomas  Murphy  brought  out 
a  complete  new  design  of  an  inclined  grate  stoker  (Fig.  13). 
This  was  probably  the  first  American  stoker  that  did  not  have 
some  resemblance  to  former  English  patents.  It,  therefore, 
stands  distinct  as  a  combination  of  elements  necessary  for  pro- 


FIG.  12. — Vicar's  Reciprocating  Bar  Stoker. 

ducing  conditions  for  proper  combustion  of  coal.  In  this  stoker, 
the  fuel  was  fed  onto  grates  from  each  side  of  the  furnace,  the 
grates  being  inclined  towards  the  middle  of  the  furnace  so  as 
to  form  a  V-shaped  cross  section.  A  combustion  arch  was  used 
extending  across  the  grates  above  the  end  of  the  V.  Arch  chan- 
nels were  provided  above  the  coking  portion  of  the  fuel  bed  for 
air  admission  for  combustion  of  volatile  gases.  The  grate  bars 


MECHANICAL  STOKERS 


27 


were  given  a  reciprocating  motion  for  breaking  up  the  caking 
fuel  bed.  Ash  and  refuse  were  disposed  of  by  means  of  a 
grinder  located  at  the  bottom  of  the  V-shaped  bars. 

Honey — 1885  (American}. — Win.  R.  Roney  brought  out  a 
front  inclined  grate  stoker  which,  although  having  the  basic 
principles  of  some  of  the  early  English  patents,  was  distinctive 
as  to  construction  details  (Fig.  14).  In  this  stoker,  coal  was  fed 
from  a  hopper  to  a  coking  plate,  and  from  there,  by  means  of 


FIG.  13. — Murphy's  Patent  Smokeless  Furnace. 

gravity  and  rocking  of  the  grate  bars,  the  fuel  bed  progressed 
to  the  bottom  of  the  furnace,  depositing  the  ash  on  the  dumping 
grates.  At  certain  intervals,  these  dumping  grates  were  dropped 
and  the  ash  and  refuse  deposited  in  a  pit. 

Bright  man — 1885  (American). — About  the  same  time  that 
the  Roney  stoker  was  brought  out,  "William  Brightman  patented 
a  front  inclined  stoker  which  was  distinctive  as  regards  to  the 
construction  of  the  grate  bars,  but  had  the  fundamental  prin- 
ciples of  the  earlier  forms  of  the  inclined  stoker. 


28  MECHANICAL  STOKERS 

Wilkinson — 1890  (American). — Wilkinson  brought  out  .a  dis- 
tinctive, front  inclined,  stoker  in  that  air  was  drawn  in  by 
steam  jets,  through  hollow  grate  bars.  It  was  designed  par- 
ticularly to  burn  small  sizes  anthracite  coals.  Coal  was  fed  onto 
the  grates  by  a  pusher  from  a  suitable  hopper.  The  grate  bars 
were  inclined  and  given  a  reciprocating  movement. 


FIG.  14 — William  R.  Honey's  Original  Stoker, 

UNDERFEED   STOKER 

Holyrod-Smith  (English). — The  first  English  patent  which 
brough  forth  the  underfeed  principle  was  that  of  Holyrod- 
Smith.  The  fuel  was  fed  from  a  hopper  into  a  horizontal 
trough  lying  across  the  front  of  the  furnace.  From  this  trough, 
the  coal  fell  into  three  longitudinal  troughs,  placed  at  right 
angles  and  passing  into  the  furnace.  By  certain  construction 
of  the  screw  conveyors  in  the  troughs,  the  coal  was  lifted  up 
into  the  burning  fuel  and  onto  perforated  side  castings  through 
which  air  was  admitted. 

Jukes — 1838  (English). — The  same  inventor  of  the  traveling 
grate  stoker  also  brought  out  a  stoker  involving  the  principle 
of  underfeeding  (Fig.  15).  Coal  from  a  hopper  dropped  in  front 
of  a  ram.  A  forward  movement  forced  the  coal  into  a  retort  and 
underneath  the  burning  fuel.  During  this  process,  the  volatile 


MECHANICAL  STOKERS 


29 


gases  were  distilled,  and  the  fixed  carbon  burned  on  fuel  sup- 
porting grates. 

Frisbie  —  1844  (American).  —  Following  the  underfeed  prin- 
ciple, Frisbie  patented  an  underfeed  stoker  in  which  fuel  was 


FIG.  15.— Jukes'  Underfeed  Stoker  of  1838. 

fed  from  beneath  through  a  central  aperture.  The  coal  was 
fed  into  a  hopper  or  box  swinging  on  pivots.  When  the  box 
was  in  place,  a  vertical  movement  of  a  plunger  pushed  the  coal 
upward  into  the  center  of  the  fuel  bed.  When  the  box  was 


FIG.  16.— The  First  Coal-burning  Jones'  Underfeed  Stoker. 

empty  and  placed  aside  for  refilling,  a  sliding  plate  closed  the 
central  aperture. 

Jones — 1889  (American). — Evan  Jones  brought  out  a  success- 
ful underfeed  stoker  which,  with  development  changes  in  details, 
is  still  being  used  successfully  (Fig.  16). 


30 


MECHANICAL  STOKERS 


This  stoker  consisted  of  a  hopper  and  plunger  for  feeding 
coal,  and  a  retort  extending  into  the  furnace.  Coal  was  forced 
forward,  by  means  of  the  piston  or  ram,  carrying  a  portion  of 
the  coal  in  the  hopper  to  the  furnace,  and  underneath  the  fuel 
bed.  Air  under  pressure  was  forced  into  a  sealed  ash  pit  from 
there  through  tuyeres  to  the  incandescent  portion  of  the  fuel 
bed. 

Wood — 1898  (American}. — W.  R.  Wood  brought  out  the 
American  stoker.  In  principle,  it  was  of  the  single  retort  type 


FIG.  17. — Early  Form  of  the  Taylor  Underfeed  Stoker. 

but  used  a  spiral  screw  for  feeding  coal  from  the  hopper  to  the 
retort  and  forcing  it  up  into  the  fuel  bed. 

Taylor — 1903  (American). — Most  of  the  early  underfeed 
stoker  inventions  provided  a  single  retort  where  the  coal  from 
the  retort  was  distributed  on  dead  plates  and  the  ash  and 
clinker  pulled  from  the  fire,  through  suitable  doors  in  the 
front.  Taylor  brought  out  an  underfeed  stoker,  which  was 
distinct,  in  that  a  series  of  inclined  retorts  were  used,  being  placed 
about  22  in.  apart  (Fig.  17).  Gravity  was  resorted  to  in  the 


MECHANICAL  STOKERS  31 

progress  of  the  fuel  from  the  coal  hopper  to  the  refuse  disposal 
plate  at  the  bottom  of  the  inclined  retorts.  Coal  was  forced  into 
the  furnace  by  plungers,  and  air  admitted  at  pressure  through 
tuyeres. 


DEVELOPMENT   OF   MECHANICAL   STOKERS 

All  of  the  early  forms  of  mechanical  stokers  showed  progress 
in  mechanically  burning  fuel,  but  still  there  were  many  operat- 
ing faults  and,  consequently,  some  skepticism  as  to  the  advis- 
ability of  installing  them.  Many  of  the  earlier  forms  of  stokers 
were  applied  to  internally-fired  boilers,  and  there  were  many 
application  problems.  In  the  United  States,  the  development 
of  the  steam  engine,  turbines  and  other  prime  movers  naturally 
required  an  improvement  in  the  character  of  firing  coal  in  order 
to  obtain  better  results.  In  the  application  of  stokers,  a  proper 
conception  of  their  scope,  benefits  and  limitation  must  be  known, 
but  in  the  device  itself  there  are  five  good  reasons  for  their 
extended  use  and  development,  these  being  (1)  To  prevent 
smoke;  (2)  To  conserve  coal;  (3)  To  produce  conditions  neces- 
sary for  the  most  economical  combustion  of  coal;  (4)  To  save 
labor,  and  (5)  To  save  equipment  and  investment. 

(1)  Smoke  Prevention. — The  factors  effecting  smoke  pre- 
vention are : 

(a)  Continuous  feed  of  coal. 

(b)  Volatile  gases  to  be  distilled  slowly. 

(c)  Mixture  of  gases  and  air. 

(d)  Proper  firebrick   combustion  chamber  for   maintaining 

proper  temperature. 

The  mechanical  stoker  now  does  all  of  these  things  well 
and  uniformly.  It  feeds  the  coal  with  mechanical  regularity 
to  a  coking  zone  of  the  fuel  bed.  The  volatile  gases  are  distilled 
slowly.  It  is  not  necessary  to  feed  coal  to  the  fires  by  open- 
ing doors  as  in  hand-firing,  resulting  in  a  reduction  of  furnace 
temperatures.  The  elimination  of  smoke  by  hand-firing  de- 
pends too  much  on  the  human  element.  The  character  of 
manual  labor  that  is  now  commercially  available  in  boiler 


32  MECHANICAL  STOKERS 

rooms  falls  considerably  short  of  the  expertness  required  for 
smoke  abatement  when  coal  is  fired  by  hand. 

(2)  Coal  Conservation. — The  principle  of  coal  conservation 
is  the  transforming  into  useful  work  every  pound  of  coal  that 
is  mined. 

The  mechanical  stoker  now  does  this  better  than  hand-firing, 
"With  the  proper  stoker,  a  poorer  and  lower  grade  of  fuel  can 
be  burned.  Years  ago,  fine  slack  coal  was  without  a  market, 
because  it  could  not  be  used  successfully  on  a  hand-fired  grate. 
The  stoker  has  been  developed  and  now  handles  this  coal  suc- 
cessfully. Many  plants  in  the  United  States  today  are  using 
the  richest  coal  (i.  e.,  high  in  heat  value,  low  in  ash  and  sulphur), 
for  industrial  purposes  on  hand-fired  grates,  when  low-grade 
coal  should  be  used  and  the  richest  coal  saved  for  other  pur- 
poses where  the  poorer  grade  fuel  cannot  be  used. 

(3)  Producing   Conditions   Necessary  for  the   most  Eco- 
nomical Combustion  of  Coal. — The  factors  effecting  the  proper 
burning  of  coal  are : 

(1)  Fuel  feed. 

(2)  Fuel  burning. 

(a)  Structure. 

(b)  Air  admission. 

(3)  Ash  disposal. 

The  mechanical  stoker  has  been  developed  to  feed  the  fuel 
to  the  furnace  by  means  of  plungers  or  pushers  with  mechani- 
cal regularity.  In  this  way,  the  volatile  gases  are  distilled 
slowly,  and  there  is  sufficient  time  to  admit  the  proper  air  for 
combustion.  In  a  properly-designed  stoker,  the  fuel  bed  has 
a  progressive  movement  by  means  of  traveling  grates  or  mov- 
able grates,  and  it  is  not  necessary  to  open  doors  to  rake  the 
fires.  Air  can  be  admitted  at  different  zones  of  the  fuel  bed 
to  burn  the  remaining  combustibles. 

The  mechanical  stoker  meets  load  conditions  better  than 
hand-firing,  for  the  reasons  that  the  air  admission  can  be 
adjusted  to  suit  fuel  bed  conditions,  and  the  rate  of  fuel 
feeding  can  be  altered  to  meet  rapid  required  changes  »n 
steam  demands.  In  the  stoker,  the  fires  are  either  cleaned 
continuously  or  by  dropping  ash  at  the  rear  of  the  grates 
periodically. 


MECHANICAL  STOKERS  33 

(4)  Labor  Saving. — The  factors  effecting  labor  saving  are : 

(a)  Installation  of  machinery. 

(b)  Intelligent  supervision. 

In  any  industry,  labor  is  saved  by  the  introduction  of 
machinery.  With  the  introduction  of  mechanical  stokers,  labor 
is  saved  just  as  soon  as  the  requirements  of  the  plant  exceed 
the  amount  of  coal  that  one  man  can  fire  by  hand.  The  amount 
that  labor  can  be  reduced  depends  on  the  size  of  the  plant. 

A  greater  saving  is  made  possible  by  the  introduction  of 
stokers,  together  with  coal  and  ash-handling  machinery.  In 
one  installation,  twelve  boilers  were  hand-fired  by  eight  fire- 
men and  a  water  tender.  With  the  installation  of  stokers, 
coal  and  ash-handling  machinery,  this  was  reduced  to  two  men 
and  a  water  tender. 

The  installation  of  mechanical  stokers  generally  results  in 
the  use  of  a  better  class  of  labor  in  the  boiler  room,  and  con- 
sequently, a  more  intelligent  supervision  of  that  labor. 

(5)  Saving  in  Equipment   and  Investment. — The   factors 
effecting  a  saving  in  boiler-room  equipment  are : 

(a)  Capacity  of  equipment. 

(b)  Flexibility  of  equipment. 

(c)  Economy  of  equipment. 

With  the  introduction  of  modern  mechanical  stokers,  coal 
can  be  burned  at  rates  of  60  to  70  Ibs.  per  sq.  ft.  of  grate  area 
per  hour,  and  this  can  be  done  with  little  loss  in  efficiency. 
This  is  impractical  with  hand-firing  which  rarely  exceeds  25 
Ibs.  per  sq.  ft.  per  hour  and  means  that  with  the  installation 
of  stokers,  the  number  of  boilers  in  a  particular  installation 
can  be  reduced  when  compared  with  a  hand-fired  installation. 

In  order  to  meet  the  sudden  demands  for  steam,  it  is  only 
necessary  to  increase  the  coal-burning  rate  of  the  stoker 
equipment  already  in  service.  With  hand-firing  which  does  not 
have  this  degree  of  flexibility,  it  is  necessary  to  put  additional 
boilers  on  the  line,  and  this  cannot  be  done  in  time  to  meet 
sudden  demands  for  steam. 

With  modern  stokers,  boilers  can  be  operated  from  50%  to 
300 %  of  boiler  rating  within  a  range  of  15%  efficiency.  This 
flexibility  is  not  obtained  with  hand-firing. 


34  MECHANICAL  STOKERS 


PRESENT   TYPES    OF    MECHANICAL    STOKERS    IN    THE    UNITED 

STATES 

The  principal  stokers  now  in  use  in  the  United  States  can 
be  divided  into  the  Chain  Crate,  Overfeed  and  Underfeed 
types.  The  names  of  the  most  prominent  stokers  are : 

Traveling  or  Chain   Grate 

B.  &  W Manufactured  by  Babcock  &  Wilcox  Co. 

Burke    "  "   Burke  Furnace  Co. 

Coxe     "  ' '   Combustion  Engineering  Corp. 

Green ' '  "   Green  Engineering  Co. 

Harrington     ' '  "   James  A.  Brady  Foundry  Co. 

Illinois    "  "   Illinois  Stoker  Co. 

LaClede-Christy    ' '  ' '   LaClede  Christy  Clay  Prod.  Co. 

McKenzie    "  ' '   McKenzie  Furnace  Co. 

Playf ord "  "   Eosedale  Foundry  &  Mach.   Co. 

Stowe    "  "   LaClede  Christy  Clay  Prod.  Co. 

Westinghouse     "  "   Westinghouse  Elec.  &  Mfg.  Co. 

Overfeed 

Detroit    Manufactured  by  Detroit  Stoker  Co. 

Model ' '  "   Automatic  Furnace  Co. 

Murphy   "  "   Murphy  Iron  Works 

Eoney    ' '  "   Westinghouse  Elec.  &  Mfg.  Co. 

Wetzel "  "   Wetzel  Stoker  Co. 

Underfeed    (Multiple  Retort) 

Jones  A.  C Manufactured  by  Underfeed  Stoker  Co.  of  America 

Eiley   "  ' '   Sanf ord-Eiley  Stoker  Co. 

Taylor   "   American  Engineering  Co. 

Westinghouse     "  ' '   Westinghouse  Elec.  &  Mfg.  Co. 

Underfeed  (Single  Eetort) 

Jones Manufactured  by  Underfeed  Stoker  Co.  of  America. 

Type   ' '  E "    "  "   Combustion  Engineering  Corp. 

Eoach    ' '  "   Eoach  Stoker  Co. 

Detroit    "  "   Detroit  Stoker  Co. 

Sturtevant «  <  <   B.  F.  Sturtevant  Co. 

CHAIN   GRATE   STOKERS 

Babcock  &  Wilcox  Chain  Grate.— This  stoker  operates  on 
natural  draft.  The  chain  (Fig.  18)  is  made  up  of  common  and 


MECHANICAL  STOKERS  35 

driving  links  about  9  in.  long  with  vertical  air  spaces.  The  links 
are  spaced  and  held  together  by  a  steel  rod  passing  through 
solid  hubs.  The  chain  passes  over  sprocket  wheels  at  the  front 
and  rear  of  the  grate,  these  being  keyed  to  steel  shafts.  The 
shafts  run  in  cast-iron  bearings  mounted  in  rectangular  guides 
at  the  rear  of  the  cast-iron  side  frames.  Adjustment  is  pro- 
vided by  means  of  screws  for  both  front  and  rear  bearings. 
Such  adjustment  makes  possible  the  taking  up  of  sag  in  the 
chain. 

The  upper  part  of  the  chain  is  supported  on  rollers  spaced 
9  inches  apart,  and  the  lower  portion  on  rollers  spaced  18  inches 


FiG.  18.— Babcock  &  Wilcox  Chain  Grate  Stoker. 

apart.  These  rollers  are  of  wrought-iron  pipe,  to  the  ends  of 
which  cast-iron  bushings  are  fitted.  The  bushings  run  on 
stationary  wrought-steel  axles  extending  from  side  to  side  on 
the  grate  and  supported  by  the  cast-iron  frames. 

The  side  frames  are  flush  with  the  top  of  the  grate.  The 
inner  sides  of  the  flush  portions  form  a  guide  against  which  the 
side  links  of  the  chain  rub.  At  the  outer  edges  of  the  side 
frames,  side  seals  are  provided  for  exclusion  of  air  at  the  sides 
of  the  furnace.  These  seals  are  held  by  weighted  levers  against 
the  under  surface  of  cast-iron  side  plates  which  are  built  into 
the  brickwork  and  overhang  the  side  frames  of  the  stoker. 

The  side  frames  are  maintained  at  a  proper  distance  from 
each  other  by  means  of  steel  spacing  bolts  front  and  rear,  and 


36  MECHANICAL  STOKERS 

a  cast-iron  cross  beam  at  the  front,  a  wrought-steel  channel 
at  the  front,  a  second  wrought-steel  channel  between  the  chains 
at  the  front,  and  a  third  wrought-steel  channel  between  the 
chains  at  the  rear.  Diagonal  rods  from  side  to  side  maintain 
the  frames  at  right  angles  to  the  cross  ties  and  shafts.  The 
frame  is  mounted  on  four  track  wheels  of  cast  iron,  18  inches 
in  diameter,  running  on  steel  axles  so  that  the  stoker  can  be 
withdrawn  from  the  furnace.  The  chain  is  about  level  from 
front  to  rear. 

This  last  wrought-steel  channel  forms  a  part  of  the  baffle 
at  the  rear  of  the  stoker  for  excluding  the  air  from  this  space. 
Below  the  channel  a  steel  plate  baffle  forms  an  additional 
spacing  piece.  Hinged  to  the  bottom  of  this  stationary  baffle 
plate  a  swinging  steel  plate,  stiffened  by  angles,  extends  to  the 
bottom  of  the  ashpit  between  the  stoker  rails,  completing  the 
seal  at  the  rear. 

A  coal  hopper  is  formed  at  the  front  by  the  cast-iron  hopper 
ends  and  by  an  inclined  steel  plate. 

A  coal  gate,  sliding  vertically  in  removable  guides  bolted 
to  the  inner  surface  of  the  cast-iron  hopper  end  pieces,  furnishes 
a  method  of  regulating  the  thickness  of  the  fuel  bed  as  fed 
to  the  forward  end  of  the  grate.  The  height  of  this  gate  is 
regulated  by  a  hand  wheel  through  a  worm  wheel  and  cross 
shaft,  which  raises  or  lowers  the  chains  from  which  the  gate 
is  hung.  The  inner  surface  of  this  gate  is  lined  with  special 
shaped  firebrick. 

The  front  sprocket  shaft  is  driven  by  a  cast-iron  worm  wheel. 
This  worm  wheel  engages  a  cast-iron  worm  secured  to  a  worm 
shaft  on  the  inner  end  of  which  is  keyed  one  of  a  pair  of 
mitre  gears.  Another  mitre  gear  which  engages  this  is  actuated 
by  a  ratchet  wheel.  Long  and  short  tool  steel  pawls  drive 
this  ratchet  wheel  from  a  cast-iron  ratchet  arm.  A  second 
pair  of  tool  steel  pawls  prevents  the  ratchet  wheel  from  mov- 
ing backward. 

The  driving  mechanism  is  encased  with  a  cast-iron  hous- 
ing. 

The  ratchet  arm  is  driven  from  an  eccentric  rod,  the  radius 
of  whose  attachment  to  the  ratchet  arm  may  be  changed  to 
increase  or  decrease  the  amount  of  feed  for  each  revolution 


MECHANICAL  STOKERS  37 

of  the  eccentric.  A  spring  safety  stop  in  the  eccentric  rod 
limits  the  power  which  may  be  transmitted  from  the  eccentric, 
to  prevent  breakage  in  case  any  foreign  object  blocks  the 
motion  of  the  stoker. 

A  stoker  water  box  is  a  part  of  the  standard  stoker  equip- 
ment. At  the  bridgewall,  a  bridgewall  water  box  of  forged 
steel  T1/^  in.  square  outside,  is  carried  transversely  across  the 
end  of  the  stoker.  The  water  box  acts  as  an  air  seal  at  this 
point. 

The  water  box  is  connected  into  the  circulation  of  the  boiler 
by  boiler  tubes  expanded  into  counterbored  seats. 

A  sprung  arch,  made  up  of  standard  firebrick  shapes,  is 
generally  used  in  connection  with  this  stoker. 

Burke  Traveling  Grate. — This  stoker  uses  natural  draft  for 
its  operation.  The  grates  are  made  up  on  a  bar  which  are 
connected  at  the  end  to  endless  chains  that  operate  around 
sprockets  at  the  front  and  rear  of  the  stoker. 

The  hopper  ends  are  supported  in  front.  An  adjustable 
gate  at  front  gives  the  desired  thickness  of  fuel  bed. 

The  front  sprockets  are  driven  by  a  nest  of  spur  gears — 
which  are  operated  from  a  line  shaft. 

Ignition  arches  are  used  with  this  stoker  as  well  as  water 
backs  at  the  rear. 

Coxe  Traveling  Grate. — This  Stoker  is  designed  to  burn 
the  small  sizes  of  anthracite  coal.  (Fig.  19.) 

The  fuel  supporting  surface  consists  of  a  number  of  parallel 
grate  bars  which  are  connected  to  endless  chains  forming  an 
endless  moving  grate.  Provision  is  made  for  introducing  air 
under  pressure  under  the  grate  with  means  for  proportioning 
the  volume  and  pressure  of  air  under  different  parts  of  the 
fire  as  may  be  necessary. 

Ignition  and  combustion  arches  are  used  over  the  grate  to 
insure  ignition. 

Coal  is  fed  to  a  hopper  extending  across  the  front  end  of 
the  stoker  above  the  grate  from  which  it  is  deposited  on  the 
grate,  the  thickness  of  the  fuel  in  the  grate  being  regulated 
by  an  adjustable  coal  gate.  Ignition  takes  place  and  combus- 
tion is  supported  by  forced  draft  under  the  grates.  There  are 
three  or  four  air  compartments  (Fig.  20)  each  extending  cross- 


38 


MECHANICAL  STOKERS 


wise  of  the  furnace,  in  each  of  which  the  air  pressure  may  be 
independently  regulated. 

The  stoker  weighs  approximately  450  Ibs.  per  square  foot 
of  active  grate  surface.    The  side  frames  on  which  practically 


FIG.  19.— End  View  of  Coxe  Traveling  Grate  Stoker. 


FIG.  20. — Elevation  and  Perspective  of  Coxe  Traveling  Grate  Stoker. 

the  entire  weight  of  the  stoker  is  carried  are  iron  castings 
of  channel  cross-section  about  34  inches  high  with  four-inch 
flanges.  These  frames  carry  on  each  end  the  shaft  bearings  or 
boxes  for  the  driving  shaft  at  the  rear  end  and  idler  shaft  at 
the  front  end. 


MECHANICAL  STOKERS 


39 


Access  to  each  tuyere  box  is  made  through  a  cast  iron  door 
located  outside  of  boiler  setting. 

The  fuel  supporting  surface  is  made  up  of  keys  or  grate 
tops  which  are  small  castings  approximately  %"  wide,  8"  long  and 
2"  deep.  The  top  surface  is  curved,  and  the  front  end  of  each 
key  matches  the  real  end  of  the  next  key. 

The  chains  are  made  of  drop  forgings  held  by  steel  pins. 

The  chains  are  carried  over  sprockets  at  the  front  and  rear 
ends  of  stoker  returning  under  the  floor  of  the  air  compart- 
ments. The  rear  shaft  is  the  driving  shaft  and  extends  through 


FIG,  21.— Green's  "  K  "  Type  Chain  Grate  Stoker. 

the  side  wall  of  boiler  where  it  is  keyed  to  a  cast  iron  worm 
wheel  mounted  in  a  cast  iron  enclosed  gear  case. 

Green  Chain  Grate. — Open  hub  links  with  vertical  air  spaces 
are  used  on  this  stoker  type  "K"  (Fig.  21)  in  making  up  the 
chain  except  those  links  which  hold  the  chain  together.  Flat- 
tened cross  bars  are  used  instead  of  round  and  the  link  designed 
so  that  if  the  rod  is  turned  on  its  edge  the  links  can  be  removed 
individually. 

The  chain  passes  over  sprockets  at  the  front  and  rear; 
adjustment  for  the  chain  being  provided  for  at  the  rear. 

The  upper  part  of  the  chain  is  supported  by  pipe  rollers 
which,  in  turn,  are  supported  by  the  side  frames.  The  upper 
edges  of  these  frames  are  set  well  below  the  top  of  the  links, 


40  MECHANICAL  STOKERS 

this  design  being  used  to  keep  the  frame  away  from  the  high 
furnace  temperatures. 

Air  seals  are  provided  at  the  rear  by  accumulation  of  ashes 
on  the  rear  girder.  This  girder,  in  connection  with  the  pipe 
rollers,  form  the  air  seal. 

The  side  frames  are  maintained  in  their  proper  position  by 
spacing  beams,  one  at  the  front  and  one  at  the  rear.  In  all 
wide  stokers,  a  center  support  is  used  for  the  front  and  rear 
sprocket  shafts  and  the  upper  roll  shafts. 

Diagonal  bars  are  used  to  keep  the  frames  at  right  angles. 
The  frame  is  mounted  on  four  track  wheels  so  the  stoker  can 
be  withdrawn  from  the  furnace. 

The  coal  hopper  is  formed  of  cast  iron  hopper  ends  and  a 
sheet  steel  front  plate.  The  feed  gate  is  supported  from  a 
square  shaft  and  moves  between  vertical  guides,  and  is  adjusted 
by  a  worm  and  sector  attachment  on  the  hopper  end.  The 
inner  surface  of  the  gate  is  lined  with  firebrick  tile  of  special 
shape,  and  designed  so  that  each  tile  can  be  removed  individu- 
ally. 

The  front  sprocket  shaft  is  driven  by  a  ratchet,  cast  steel 
pawl  and  cast  steel  gear  train,  babbitted  in  a  self-contained 
frame  which  is  bolted  to  the  stoker  front  side  frames.  The 
ratchet  is  operated  from  an  eccentric  on  the  main  driving  shaft, 
the  eccentric  rod  being  provided  with  a  safety  spring  in  case 
the  grate  is  blocked  for  any  reason  in  its  motion. 

A  high  pressure  water  box  is  used  with  the  stoker  at  the 
bridgewall.  The  water  box  is  connected  into  the  circulation 
of  the  boiler  by  boiler  tubes. 

An  ignition  arch  of  the  suspended  type  is  used  at  the  front 
of  the  stoker;  specially  formed  firebrick  tile  being  suspended 
from  I-beams. 

The  stoker  operates  on  natural  draft. 

"L"   TYPE   GREEN  STOKER 

This  company  also  make  the  "L"  Type  stoker  (Fig.  23) 
the  important  difference  from  the  "K"  Type  being  a  fuel 
pusher  and  inclined  coking  plates  in  front  of  the  chain  grate. 
These  inclined  plates  are  designed  to  keep  fuel  broken  up 


MECHANICAL  STOKERS 


41 


during  the  first  period  of  burning,  and  delivering  it  to  the 
chain  without  large,  unmanageable  masses  of  coke.  After  the 
coal  is  coked  on  the  front  part  of  the  stoker  then  combustion 
is  completed  upon  the  plain  chain  grates. 

The  Fuel  Pusher  is  made  as  wide  as  the  hopper.  It  is 
operated  through  an  adjustable  stroke  giving  a  positive  means 
of  controlling  the  fuel  bed.  In  this  way  the  fuel  bed  is  varied. 
The  driving  mechanism  of  the  pusher  plates  is  independent  of 
the  chain  drive  and  of  the  coking  plate  agitators. 


FIG.  22.— Front  View  Green  "  L  "  Type  Chain  Grate  Stoker, 

To  maintain  uniform  density  in  the  fuel  bed  during  the 
coking  process,  the  fuel  pushers  are  provided  with  small  adjust- 
able sections  each  12"  wide. 

The  inclined  coking  plates  form  an  inclined  grate  area 
between  the  fuel  hopper  and  the  chain  grate.  Agitation  is 
given  to  the  coking  plates  to  keep  the  fuel  particles  in  motion. 

The  rear  cross  girder  of  this  stoker  is  a  semi-steel  box 
maintained  at  low  temperature  by  water  circulation.  It  is  de- 
signed to  hold  the  stoker  frame  square  and  true. 

The  chain  is  kept  at  tension  by  worm  gearing  in  cast  iron 


42 


MECHANICAL  STOKERS 


casings.  These  are  interconnected  and  can  be  adjusted  from 
the  side.  A  water  cooled  surface  is  placed  in  the  wall  adjacent 
to  the  coking  plates  to  prevent  clinker  adhesions  and  erosion 
of  brick  work. 

A  coking  or  ignition  arch  is  used  over  the  front  part  of  the 
stoker.  This  arch  is  of  special  design  of  suspended  type.  High 
pressure  water  backs  are  used  at  the  rear — connected  up  to  the 
boiler  circulation. 

The  stoker  can  be  operated  on  either  natural  or  forced 
draft. 


FIG,  23,— Green's  "  L  "  Type  Chain  Grate  Stoker. 

Illinois  Chain  Grate.— Natural  draft  is  used  with  this  Type 
"A"  stoker  (Fig.  24).  The  links  are  about  9"  long,  solid,  and 
held  together  by  link  rods.  The  spaces  in  the  link  through 
which  air  passes  to  the  fuel  bed  are  placed  at  an  angle  and 
when  the  chain  is  assembled  the  angles  of  adjacent  links  cross 
each  other.  This  design  has  been  used  in  an  attempt  to  reduce 
the  sifting  of  fine  coal,  through  the  air  spaces,  to  a  minimum. 
The  chain  is  slightly  inclined  from  front  to  rear. 

The  chain  passes  over  sprocket  wheels  at  the  front  and  rear. 
The  rear  sprockets  are  loose  on  the  shaft.  The  front  driving 
sprockets  engage  small  rollers  that  are  placed  in  between  the 


MECHANICAL  STOKERS 


43 


links.  This  construction  differs  from  designs  where  the  driv- 
ing sprockets  engage  the  links.  Adjustment  of  the  chain  is 
provided  by  means  of  take-up  screws  at  the  front  of  the  stoker. 
The  rollers  upon  which  the  chains  travel  are  spaced  about 
12  in.  apart,  the  roller  rods  being  supported  by  side  frames  which 
are  cast  in  a  single  piece,  that  is,  the  hopper  supports  and  rear 
frames  are  one  piece.  The  frames  are  held  together  by  two 
cast-iron  girders,  one  at  each  end,  and  diagonal  tie  rods  from 
side  to  side  maintain  the  frames  at  right  angles.  Air  seals 
are  placed  at  the  rear  of  the  frames. 


FIG.  24.— Type  "  A  "  Illinois  Chain  Grate  Stoker. 

The  hopper  ends  are  supported  from  the  side  frames,  as 
shown  in  Fig.  25.  The  adjustable  gate  to  give  the  desired 
thickness  of  fuel  bed,  is  operated  by  a  hand  wheel  at  the  side 
of  the  stoker  which  is  connected  to  a  shaft  extending  across 
the  stoker  by  means  of  a  worm  and  gear  design.  The  gate  is 
gradually  raised  or  lowered.  The  inner  surface  of  the  gate  is 
lined  with  special-shape  firebrick. 

The  front  sprockets  are  driven  by  a  worm  which  engages 
a  pawl  and  ratchet  wheel  operated  by  the  eccentric  on  the  drive 
shafting.  The  speed  of  the  grate  is  varied  from  one  to  five 
inches  per  minute  by  moving  a  hand  lever  forward  or  back- 
ward, thus  controlling  the  number  of  teeth  on  the  ratchet 
wheel  that  the  pawl  engages  at  each  stroke. 


44 


MECHANICAL  STOKERS 


A  low  pressure  water  box  made  up  of  wrought  iron  pipe  is 
generally  used  at  the  bridgewall. 

A  suspended  fire  brick  arch  is  used,  made  up  of  special 
shaped  blocks  and  suspended  over  the  stoker. 

This  company  also  manufactures  a  Type  "G" — (Fig.  25) 
forced  draft  chain  grate  stoker.  The  important  feature  of 
which  is  the  system  of  dampered  air  control.  Air  required 
for  combustion  is  delivered  to  a  wind  box  which  is  incor- 
porated in  the  side  wall  of  the  setting.  The  dampers  for  air 
admission  to  different  parts  of  the  grate  surface  are  operated 


J 


FIG.  25.— Type  "  G  ';  Illinois  Forced  Draft  Chain  Grate  Stoker. 

from  the  side.  The  wind  box  is  usually  made  of  concrete  and 
is  the  base  of  the  side  wall.  Baffles  are  used  so  that  air  cannot 
pass  through  to  the  end  of  the  grate.  The  mechanical  opera- 
tion of  this  stoker  is  practically  like  the  Type  "A"  stoker, 
having  sprockets  in  front — which  are  connected  to  a  worm 
wheel  and  worm  attached  to  the  end  of  the  front  sprocket 
shaft.  Rear  drums  are  used  similar  to  the  Type  "A"  stoker. 
An  arch  is  used  for  ignition  purposes. 

Laclede- Christy  Chain  Grate. — The  links  of  this  stoker 
(Fig.  26),  are  all  of  the  same  design  with  the  exception  of 
the  side  links.  They  are  about  9"  long  with  the  air  spaces  at 
an  angle.  Cast  iron  rollers  are  placed  on  the  link  rods  in 


MECHANICAL  STOKERS  45 

between  the  links  and  act  as  spreaders,  and  also  engage  the 
front  sprockets.  The  chain  in  slightly  inclined  from  front  to 
rear. 

The  chain  is  driven  by  the  front  sprockets,  keyed  onto  the 
shaft.  The  rear  end  of  the  chain  passes  over  idler  pulleys 
which  are  loose  on  the  shaft.  Take-up  boxes  are  located  at  the 
front  of  the  stoker. 

The  side  frames  are  made  of  two  pieces,  the  front  part 
being  bolted  to  the  rear  part.  These  side  frames  are  held 
together  by  rods  and  pipe  spacers  and  diagonal  bracing,  the 


FIG.  26.— Laclede-Christy  Chain  Grate  Stoker. 

chain  being  supported  on  the  pipe  rollers,  one  at  the  top  of 
the  frame  and  one  at  the  bottom.  The  entire  frame  is  sup- 
ported on  four  cast-iron  flanged  wheels  which  fit  T-rails  so 
that  the  stoker  can  be  withdrawn. 

The  feed  gate  is  adjusted  by  two  hand  wheels,  one  on  each 
side  of  the  stoker,  and  regulate  the  thickness  of  the  fuel  bed 
from  V  to  10".  The  inner  surface  of  the  gate  is  lined  with 
special  shape  firebrick  blocks  having  rounded  ends  where  the 
coal  is  admitted. 

Air  seals  are  placed  near  the  rear  idler  wheels  and  a  second 
damper  which  can  be  regulated,  placed  24"  to  the  front  of  the 
rear  seal. 


46  MECHANICAL  STOKERS 

The  driving  mechanism  consists  of  a  worm  gear  operated 
by  a  pawl  and  ratchet  which  can  be  regulated  to  control  the 
speed  of  the  grate.  The  pawl  is  operated  by  an  eccentric 
placed  on  the  driving  shaft. 

A  low  pressure  water  box  at  the  bridgewall  and  a  sus- 
pended arch  of  special  firebrick  shapes  is  generally  used. 
Natural  draft  is  used  with  this  stoker. 

Continental  Traveling  Grate. — The  chain  of  this  stoker 
(Fig.  27),  is  distinctive  in  that  grates  are  not  interlocking  as 
the  ordinary  link  design.  Natural  draft  is  used. 

The  grates  are  designed  and  constructed  of  small  units,  with 
dovetail  and  semicircle  for  locking  each  grate,  this  dovetail 


FIG.  27. — Continental  Chain  Grate  Stoker. 

being  inserted  into  the  openings  in  the  links  and  bars,  a  rod 
passing  through  the  cored  hole  locks  each  grate  into  its  proper 
position.  The  grate  can  be  removed  and  replaced,  there  being 
one  rod  which  locks  each  section  of  grate  in  its  position.  This 
rod  acts  as  a  key  to  each  grate.  The  grates  are  interchange- 
able and  of  one  size. 

An  independent  chain  is  used  of  links  separated  and  bolted 
to  cast-iron  bars.  These  bars  act  as  supports  for  grates. 

The  frame  consists  of  two  cast-iron  side  frames  connected 
to  two  hopper  frames,  braced  by  four  cross  beams  transversely 
to  the  frames,  and  two  steel  shafts  upon  which  are  mounted 
and  keyed  the  sprocket  wheels  which  impart  the  traveling 


MECHANICAL  STOKERS  47 

motion  to  the  chain.  The  frame  is  mounted  upon  flanged  wheels 
which  rest  upon  rails  set  under  the  furnace  so  that  the  stoker 
can  be  withdrawn  from  the  furnace. 

The  driving  mechanism  utilizes  the  double  acting  upward 
stroke  of  cams.  The  spur  gear,  consisting  of  a  train  of  gears, 
is  enclosed  in  a  casing.  The  gears  are  driven  by  ratchet  wheels 
and  pawls,  the  ratchet  wheels  being  operated  by  pawl  levers 
connected  to  the  speed  adjustment  links  by  rods  which  contain 
the  relief  springs.  The  adjustment  links  are  connected  to  the 
roller  levers  and  these  levers  ride  on  the  cams  of  the  line 
shaft. 

A  low  pressure  water  box  is  used  at  the  bridgewall  and  a 
flat  suspended  arch  at  the  front  of  the  stoker. 

McKenzie  Traveling-  Grate. — This  stoker  is  of  the  traveling 
grate  class,  having  the  entire  grate  surface  detachable  and 
fastened  to  bars  in  small  sections.  This  design  has  been  worked 
out  so  as  to  make  it  possible  to  remove  any  grate  bar  inde- 
pendent of  the  others. 

The  entire  stoker  is  built  up  with  side  frames  which  are 
supported  on  four  wheels  so  that  the  stoker  can  be  removed 
from  the  furnace. 

The  driving  mechanism  is  made  up  of  what  is  termed 
double-acting,  giving  a  continuous  travel  to  the  grate  surface, 
the  speed  of  the  grate  being  controlled  by  operating  mechanism 
fastened  to  the  side  of  the  stoker  part. 

The  stoker  is  also  equipped  with  a  clinker  apron  at  the 
rear  and  a  horizontal  dumping  grate,  this  dumping  grate  form- 
ing an  ash  receiver,  this  design  being  used  to  prevent,  as  much 
as  possible,  excess  air  at  the  rear  of  the  grate. 

The  stoker  is  also  equipped  with  a  flat  suspended  ignition 
arch  at  the  front  and  a  low-pressure  water  box  at  the  rear. 
Natural  draft  is  used  for  the  operation  of  the  stoker. 

Harrington  Traveling  Grate.— This  stoker  consists  of  cast- 
iron  side  frames,  carrying  the  driving  gear,  hopper  front  shaft 
and  feed  gate  in  the  usual  manner  (Fig.  28}.  The  side  girders 
are  formed  of  structural-steel  members,  built  like  a  truss. 
Transverse  members  of  structural  steel  support  a  series  of 
tracks,  on  which  run  semi-steel  chains,  which  carry  and  support 
the  grate  surface  and  take  up  the  stress  and  tension  of  the 


48 


MECHANICAL  STOKERS 


chain.  These  are  provided  with  V-rollers  to  insure  alignment 
both  horizontally  and  vertically,  and  to  reduce  the  power 
required  for  driving  the  stoker.  An  inclosed  double  worm 
drive  is  provided  at  the  rear  of  the  stoker.  Attached  to  the 
chains  is  a  series  of  transverse  racks  or  beams  on  which  the 
clips  or  bars  forming  the  grate  surface  are  attached. 

The  grate  bars  are  sufficiently  loose  to  slide  over  the  ends 
of  the  racks.  The  straight  under-surface  of  these  racks  make 
an  air-tight  diaphragm  of  seal  between  the  adjacent  compart- 


FIG.  28. — Front  View,  Showing  Hopper  Parts  of  Harrington  Forced  Draft 
Traveling  Grate  Stoker. 

ments.  These  compartments  occupy  the  space  between  the 
chains  communicating  on  one  or  both  sides  to  the  air  duct  in 
the  boiler  side  walls,  or  below  the  floor  of  the  boiler  room. 

An  adjustable  damper  serves  to  control  the  air  pressure 
in  the  respective  compartments.  Each  communicating  passage 
through  the  side  wall  terminates  in  a  removable  door,  which, 
when  taken  off,  allows  free  access  to  the  chamber.  The  clos- 
ing of  the  damper  and  the  removal  of  this  door  serves  to  put 
the  stoker  on  a  natural-draft  basis. 


MECHANICAL  STOKERS 


49 


The  grate  bars  fit  close  together,  and  the  air  in  passing 
through  them  makes  two  right-angle  turns.  The  lower  shoulder 
at  the  joint  is  designed  to  prevent  the  falling  of  fuel  through 
the  grate.  Projections  hold  the  adjacent  surfaces  apart  so 
that  an  air  space  of  approximately  15%  is  attained. 

Fig.  29  is  a  view  of  the  stoker  partly  assembled.  It  has 
four  compartments,  the  one  under  the  central  part  of  the  grate 
having  double  the  width  of  any  one  of  the  others.  Under- 


FIG.  29. — Harrington  Forced  Draft  Traveling  Grate  Stoker  .Partly  Assembled. 

neath  the  lower  run  of  the  grate  an  extension  is  built  out  over 
the  ashpit  to  protect  the  bars  from  radiated  heat,  and  a  seal 
under  the  rear  cross  baffle  is  carried  up  as  close  as  possible  to 
the  moving  surface.  These  provisions  obviate  the  need  for  a 
water-back  or  an  overhang  to  the  bridge-wall. 

The  chambers  communicating  between  the  compartments 
and  the  common  air  duct  from  the  fan  are  shown  in  Fig.  29. 
The  duct  passes  immediately  below  and  a  common  damper 
plate  in  the  bottom  of  each  chamber  controls  the  volume  of  air 


50 


MECHANICAL  STOKERS 


to  each  compartment.  The  dampers  are  operated  manually  by 
means  of  the  lever  and  hand-wheel  shown.  Each  damper  is  set 
in  accordance  with  requirements  while  the  total  air  supply  is 
controlled  through  the  fan  serving  the  stoker. 

Playford  Chain  Grate. — This  stoker  (Fig.  30)  operates  on 
natural  draft.  The  chain  consists  of  a  number  of  driving  and 
standard  links  with  vertical  air  spaces,  traveling  in  the  form  of 
an  endless  chain  over  supporting  rollers  at  the  rear.  The 
motion  is  imparted  by  the  sprocket  wheels  which  are  mounted 
on  the  front  driving  shaft,  and  engage  with  the  rods  support- 
ing the  grate  links,  which  form  the  chain.  Adjustment  of  the 
chain  is  provided  by  screws  at  the  front  of  the  stoker. 


^i^^Ajgp^^^^^^     ^IHHHHHHi 


FIG.  30. — Playford  Chain  Grate  Stoker. 

The  chain  is  supported  by  pipe  rollers  fastened  to  the  side 
frames  which  are  spaced  by  the  usual  cross  beams.  Air  seals 
are  provided  at  the  rear  of  the  stoker.  The  frame  is  mounted 
on  wheels  so  it  can  be  removed  from  the  furnace. 

The  motion  of  the  driving  shaft  is  derived  from  a  ratchet, 
worm  and  gear,  located  at  the  side  of  the  stoker,  and  is  trans- 
mitted through  an  eccentric  from  the  line  shaft. 

The  coal  feed  is  regulated  by  raising  or  lowering  a  sheet 
steel  water  gate  by  means  of  a  hand  wheel  located  at  the  side 
of  the  stoker. 

A  low  pressure  water  box  at  the  bridgewall  and  an  ignition 
arch,  built  of  standard  size  firebrick  and  located  immediately 
at  the  front  of  the  stoker. 


MECHANICAL  STOKERS 


51 


Stowe  Traveling  Grate. — This  stoker  (Fig.  31)  consists  of 
traveling  grates  alternating  with  stationary  tuyeres.  The  grate 
surface  is  inclined  towards  the  bridge  wall  at  an  angle  of  twenty 
degrees.  The  stoker  essentially  is  made  up  of  coal  hopper  in 
front — where  the  coal  is  allowed  to  feed  with  the  grate  accord- 
ing to  a  thickness  set  by  a  feed  gate.  The  chain  grates  are 
about  4"  wide  and  the  tuyeres  in  between  are  about  3"  wide.  The 
top  of  the  tuyeres  are  depressed  slightly  below  the  chain.  Near 
the  bridge  wall  the  grates  rise  to  form  a  more  horizontal  grate 


FIG.  31. — Stowe  Conveyor  Feed  Stoker. 

surface — this  being  designed  to  retard  or  hold  back  the  fuel 
bed,  if  necessary,  to  burn  all  combustible  matter  left. 

The  chain  elements  are  driven  by  sprockets  keyed  to  a 
single  shaft  extending  across  the  stoker  in  front.  The  main 
sprocket  shaft  is  driven  by  an  eccentric  and  ratchet  arrange- 
ment through  a  nest  of  spur  gears. 

As  the  chain  grates  move  down  in  the  furnace  and  around 
the  sprockets  as  in  an  ordinary  chain  grate  stoker  they  are 
supported  by  cast  iron  skids.  These  skids  also  serve  as  a  sup- 
port for  the  tuyeres.  A  complete  tuyere  unit  extends  from  the 


52  MECHANICAL  STOKERS 

dead  plates  in  between  the  chain  grates  at  the  top  to  retarding 
bars  in  between  the  chain  grates  at  the  rear. 

Air  for  the  stoker  is  admitted  from  a  chamber  extending 
beneath  the  entire  grate  that  serves  as  a  wind  box;  at  the  rear 
is  a  concrete  wall  on  which  is  mounted  a  series  of  cast-iron 
tunnels  through  which  the  returning  chain  grates  pass. 

A  suspended  arch  is  used  with  this  stoker  for  ignition  pur- 
poses— as  coal  is  burned  by  the  over  feed  method  the  same  as  a 
standard  chain  grate.  A  water  back  is  not  used  however  but 
if  the  ash  pit  is  not  sealed  it  would  be  necessary  in  order  to 


FIG.  32. — Westinghouse  Chain  Grate  Stoker. 

eliminate  air  leakage  between  the  rear  of  the  stoker  and  bridge 
wall. 

Westinghouse  Chain  Grate. — The  chain  (Fig.  32}  is  made 
up  of  standard  and  driving  links  with  solid  hubs  9"  long 
weighing  6  Ibs.  and  8  Ibs.,  respectively.  They  are  spaced  and 
held  together  by  steel  rods. 

The  chain  passes  over  sprockets  at  the  front  and  rear.  The 
front  sprockets  are  keyed  to  the  shaft.  The  rear  sprockets 
are  all  connected  together  by  pipe  spacers  and  through  bolts, 
and  the  entire  unit  revolves  on  a  steel  shaft  supported  by  cast- 
iron  bearings  mounted  in  the  side  frames. 

The  upper  part  of  the  chain  is  supported  on  pipe  rollers 
with  cast-iron  ends  or  bushings.  These  bushings  run  on  steel 


MECHANICAL  STOKERS  53 

cross  rods,  spaced  about  9  inches  apart.  The  lower  portion  of 
the  grate  is  supported  on  pipe  rollers  spaced  about  18  inches 
apart. 

The  side  frames  are  in  one  piece  with  removable  top  pieces 
that  come  flush  with  the  top  of  the  links.  The  frames  are  held 
together  by  a  cast-iron  beam  at  the  front  and  a  wrought-iron 
channel  at  the  rear.  Diagonal  rods  are  also  used. 

The  frame  is  mounted  on  four  wheels  of  cast  iron,  so  the 
stoker  can  be  withdrawn  from  the  furnace.  The  chain  is  level 
from  the  front  to  rear.  Air  seals  are  provided  at  the  reai* 
cross  beam. 

The  hopper  is  made  up  of  hopper  ends  bolted  to  the  side 
frames  and  a  steel  front  plate.  The  coal  gate  is  supended  from 
a  channel  supported  on  the  hopper  ends  and  adjusted  by  two 
hand  wheels  on  each  side  of  the  gate.  The  inner  surface  of 
the  gate  is  lined  with  standard  arch  and  straight  firebrick 
held  together  by  cast  iron  clamps. 

The  chain  is  driven  by  a  cast-iron  worm  gear  which  engages 
a  cast-iron  worm.  Two  mitre  gears  are  actuated  by  a  ratchet 
which,  in  turn,  is  driven  by  a  pawl  placed  between  two  rods 
making  up  the  ratchet  arm.  The  ratchet  arm  is  driven  from 
an  eccentric  placed  on  the  driving  shaft.  A  safety  spring  is 
placed  in  the  eccentric  rod  to  prevent  breakage. 

A  4"  wrought-iron  low-pressure  water  box  is  used  at  the 
bridge  wall,  and  a  sprung  arch  of  standard  firebrick  at  the 
front  of  the  stoker. 

Natural  draft  is  used  with  this  stoker. 


OVERFEED  TYPE  STOKERS 

Cox-Fulton  Stoker.— This  stoker  is  of  the  front  feed  class 
operating  on  natural  draft  (Fig.  33).  The  coal  hopper  extends 
across  the  front  of  the  stoker.  The  coal  is  fed  to  the  coking 
plate  and  onto  the  inclined  grate  bars  by  a  reciprocating 
pusher,  operated  by  a  sector  on  the  hopper  shaft.  Motion  is 
given  to  the  hopper  shaft  by  means  of  an  eccentric  on  the 
main  shaft.  The  motion  of  the  pusher  is  adjustable.  The  grate 
supports  are  inclined  to  the  rear  about  39°. 

The  grate  bars  are  water-cooled  on  their  inner  edges,  steel 


MECHANICAL  STOKERS 


tubing  being  used  to  connect  the  grates  to  the  water  supply. 
Movable  and  stationary  grates  are  used,  giving  a  forward- 
upward-inward  motion  to  the  movable  grates.  Motion  to  the 
grates  is  obtained  from  a  main  shaft  located  in  front  of  the 
stoker  through  eccentrics  connected  to  inclined  grate  bar 
bearers.  The  movement  of  the  grates  forces  the  fuel  bed  to  the 
bottom  of  the  furnace  on  the  dump  grate. 


FIG.  33. — Cox-Fulton  Inclined  Overfeed  Stoker. 

Dump  grates  and  guards  are  operated  from  the  front,  and 
thus  the  ash  and  refuse  is  dumped  into  the  ash-pit  periodically. 
An  ignition  arch  of  the  sprung  or  suspended  type  is  used. 

Model  Stoker. — This  stoker  is  of  the  side  feed  class  having 
the  coal  hoppers  on  the  side  and  operated  on  natural  draft 
of  from  .25"  to  .6"  in  the  furnace.  The  grates  are  ar- 
ranged in  pairs  inclined  from  the  sides  to  the  center  in  a 
V-shape,  one  of  each  pair  on  each  side  being  stationary  and 
the  other  movable.  When  in  operation,  the  movable  grate  is 
moved  by  the  rocker  bar  up  so  its  fire  edge  is  a  little  above 
the  fire  edge  of  the  stationary  grate  and  then  down  a  little 
below.  The  stationary  grates  rest  at  the  lower  end  on  the 


MECHANICAL  STOKERS  55 

bearer  and  at  the  upper  end  against  the  edge  of  the  bottom 
plate  of  the  magazine  over  which  the  coal  is  fed  into  the 
furnace.  The  movable  grates  are  hinged  to  the  stationary 
grates  by  a  pin  lug  which  fits  into  a  hole  in  the  stationary 
grate  near  its  upper  end,  the  lower  end  of  the  movable  grate 
being  held  to  place  and  rocked  by  the  rocker  bar  as  indicated. 


FIG.  34. — Murphy  Automatic  Furnace. 

The  grate  bearer  located  in  the  center  of  the  furnace  sup- 
ports the  grates  and  forms  a  box-like  receptacle  into  which  the 
clinkers  and  ash  pass  and  are  ground  out  into  the  ash  pit. 

A  sprung  arch  is  used  over  the  V-shaped  grates,  and  air 
admitted  at  the  point  where  the  volatiles  are  distilled  from 
the  coal  as  it  comes  from  the  stoker  coal  hoppers. 

Murphy  Stoker. — The  Murphy  stoker  is  of  the  side  feed 
class,  having  the  coal  hoppers  on  the  sides  with  inclined  fires 


56  MECHANICAL  STOKERS 

on  both  sides  of  the  furnace  and  the  observation  door  and 
operating  mechanism  in  the  front  (Fig.  34).  This  stoker  uses 
natural  draft. 

At  either  side  of  the  furnace,  extending  from  front  to 
rear,  is  a  coal  magazine  into  which  the  coal  is  introduced  by 
conveyors.  At  the  bottom  of  this  magazine  is  the  coking  plate 
against  which  the  inclined  grates  rest  at  their  upper  ends. 
The  stoker  boxes,  operated  by  segment  gear  shaft  and  racks, 
push  the  coal  out  over  the  coking  plate  and  onto  the  grates. 

The  grates  are  made  in  pairs,  one  fixed,  the  other  movable. 
The  movable  grates,  pinioned  at  their  upper  ends,  are  moved 
by  a  rocker  bar  at  their-  lower  ends,  alternately  above  and 
below  the  surface  of  the  stationary  grates.  The  stationary 
grates  rest  upon  the  grate  bearer  which  also  contains  the 
clinker  or  ash  grinder.  This  grate  bearer  is  cast  hollow  and 
receives  the  exhaust  steam  from  the  stoker  engine.  This  steam 
escapes  through  small  openings  spaced  at  regular  intervals 
on  either  side  of  the  clinker  grinder  and  lower  ends  of  the 
grates  to  soften  the  clinker  and  to  assist  the  cleaning  process. 

As  the  coal  leaves  the  magazine,  it  rests  upon  the  coking 
plate.  The  volatile  gases  are  driven  off  and  mixed  with  the 
heated  air  admitted  through  the  air  ducts  in  the  arch  plate. 
The  coal,  having  been  coked,  travels  down  the  inclined  grates 
toward  the  clinker  grinder,  receiving  air  in  the  ordinary  way 
through  the  grates  to  complete  the  burning  process. 

The  speed  at  which  the  stoker  boxes  push  the  coal  on  the 
grates  can  be  regulated  to  conform  to  the  duty  required.  Like- 
wise, the  clinker  grinder  can  be  turned  slower  or  faster  accord- 
ing to  the  amount  of  ash  in  the  coal. 

A  sprung  arch  is  used  extending  from  one  hopper  to  the 
other. 

Detroit  Stoker. — This  stoker  uses  natural  draft  and  is  of 
the  side  feed  class  having  the  coal  hoppers  on  the  sides  of  the 
stoker  front  (Fig.  35).  From  the  hopper,  the  coal  is  fed  into 
the  magazines  by  a  worm  coal  conveyor.  The  gears  operating 
the  worm  conveyors  are  lubricated  by  running  in  oil,  and  are 
covered  with  shields.  The  coal  conveyors  are  operated  by  the 
upper  shaft  in  front  at  a  slow  speed  and  distribute  the  coal 
at  the  upper  end  of  the  inclined  grates  on  a  coking  plate. 


MECHANICAL  STOKERS 


57 


Inclined  grates  of  two  kinds  are  used,  i.e.,  stationary  and 
moving,  each  alternate  grate  being  operated  by  the  driving 
shaft  in  front.  The  vibrating  or  operating  grates  have  a 
motion  forward  and  backward,  moving  the  bed  of  fire  down 
toward  the  center  of  the  furnace. 

The  vibrating  grates  are  operated  by  upper  and  lower 
rocker  bars  connected  to  the  lower  driving  shaft  by  links, 


FIG.  35 —Detroit  "  V  "  Type  Overfeed  Stoker. 

which  can  be  unhooked  during  the  operation,  thereby  discon- 
tinuing the  grate  movement  entirely  when  desired. 

In  the  center  of  the  stoker,  and  at  the  bottom  of  the  inclined 
bars,  is  a  clinker  crusher  composed  of  a  row  of  cast-iron  discs, 
ten  inches  in  diameter,  which  rotate  alternately  toward  and 
from  each  other  by  the  reverse  gear  and  is  connected  with  the 
front  driving  shaft.  This  serves  to  crush  the  clinkers  and 
deposits  them  in  the  ash  pit. 


58 


MECHANICAL  STOKERS 


A  sprung  or  suspended  arch  is  used  with  this  stoker  and 
air  admitted  through  channels  forming  the  support  for  the 
arch  skewbacks,  this  air  being  admitted  where  the  volatile 
gases  are  distilled.  An  observation  door  is  placed  in  the  stoker 
front  to  observe  conditions  of  the  furnace. 

Roney  Stoker. — The  Roney  stoker  (Fig.  36)  operates  on 
natural  draft  and  belongs  to  the  front-feed  class,  the  coal  being 
fed  from  a  hopper  at  the  front  of  the  furnace. 


FIG.  36. — Westinghouse  lloney  Stoker. 


From  the  hopper,  the  coal  is  pushed  by  an  adjustable  pusher 
onto  a  coking  plate  where  the  volatiles  are  distilled.  From 
the  coking  plate  the  fuel  passes  to  the  grates.  The  grates  con- 
sist of  cast-iron  webs  on  which  are  placed  narrow  grate  bar 
tops.  In  the  upper  part  of  the  furnace,  flat  overlapping  bars 
are  used  to  prevent  sifting  of  fine  coal.  Each  web  is  sup- 
ported at  its  ends  by  trunnions  and  is  connected  by  an  arm  to 
a  rocker  bar  which  is  slowly  moved  to  and  fro  by  an  eccentric 
on  the  shaft  on  the  stoker  front  so  as  to  rock  the  grates  back 
and  forth  between  a  horizontal  position  and  an  inclination 


MECHANICAL  STOKERS  59 

towards  the  back  of  the  furnace.  The  grates  thus  gradually 
move  the  burning  coke  downwards. 

The  ashes  and  clinkers  are  deposited  onto  the  dumping 
grate  which  can  be  lowered  by  means  of  rods  from  the  front 
of  the  stoker  so  as  to  drop  the  ash  into  the  ash-pit  below.  A 
guard  may  be  raised  so  as  to  prevent  coke  or  coal  on  the  grate 
bars  from  falling  into  the  ash-pit  when  the  dumping  grate  is 
lowered. 

Air  for  the  volatile  gases  is  admitted  in  small  jets  at  the 
front  of  the  stoker.  A  firebrick  arch  is  used  at  the  front  made 
up  of  standard  firebrick  shapes. 

The  stoker  shaft  on  the  front  of  the  stoker  is  operated  at 
7  to  14  B.  P.  M.  by  a  small  engine  through  a  worm  and  gear 
reduction.  Doors  are  provided  on  each  side  of  the  hopper  for 
observation  of  furnace  conditions. 

Wetzel  Stoker. — This  stoker  operates  on  natural  draft  and 
is  of  the  front-feed  class,  since  the  coal  is  placed  in  a  hopper 
at  the  front  of  the  furnace  (Fig.  37).  From  this  hopper,  the 
fuel  is  pushed  over  the  dead  plate  and  onto  the  coking  grate 
by  the  feeder,  moved  by  an  eccentric  on  the  shaft.  The  cokr 
ing  grate  driven  by  a  link  connection  to  the  feeder  moves  the 
coal  onto  the  main  grate. 

The  main  grate  consists  of  a  series  of  movable,  and  a  series 
of  stationary  bars,  the  bars  of  one  series  being  alternately 
arranged  with  respect  to  those  of  the  other.  The  grates  are 
mounted  on  a  stationary  cast-iron  frame,  the  stationary  grates 
being  directly  attached  thereto  and  the  movable  grates  sup- 
ported by  rock  shafts.  The  movable  bars  are  driven  by  an 
eccentric  on  the  shaft  operating  through  a  bell  crank.  The 
motion  of  the  bars  moves  the  fuel  down  the  main  grate  which  is 
inclined  at  about  30°,  and  discharges  the  ash  onto  the  dump- 
ing grate.  When  sufficient  ash  has  been  accumulated  on  the 
dumping  grate,  a  lever  is  thrown  from  the  front  of  the  furnace 
and  the  ash  is  discharged  into  the  ash  pit. 

The  coking  grate  contains  an  extremely  large  percentage 
of  air  spaces,  the  upper  part  of  the  main  grate  a  somewhat 
smaller  percentage,  the  lower  part  of  the  main  grate  still 
less,  and  the  dumping  grate  a  very  small  percentage. 


60  MECHANICAL  STOKERS 

Above  the  dead  plate,  coking  grate  and  the  upper  part  of 
the  main  grate  is  sprung  a  firebrick  ignition  arch. 

The  amount  of  motion  of  the  feeder  of  the  coal  and  motion 


FIG.  37.— Wetzel  Front  Inclined  Overfeed  Stoker. 

of  the  grates  is  adjusted  from  the  front  of  the  stoker  by  means 
of  hand-wheels  on  the  eccentric  rods. 

An  inspection  door  is  provided  at  the  front  on  each  side  of 
the  hopper  enabling  the  operator  to  view  the  condition  of  the 
fire. 

The  main  shaft  which  runs  along  the  stoker  front  below 


MECHANICAL  STOKERS 


61 


the  hopper  is  driven  by  a  steam  engine  operating  through  a 
worm  gearing. 

Wilkinson  Stoker.— The  Wilkinson  stoker  (Fig.  38)  is  of 
the  front-feed  class  and  designed  more  particularly  for  the 
burning  of  fine  anthracite  coal.  The  coal  hopper  extends 
across  the  front  of  the  stoker.  A  pusher  fastened  to  the  upper 
end  of  each  grate  bar  pushes  the  coal  from  the  hopper  through 
the  opening  in  the  furnace  front  onto  the  bars.  The  grates 
are  inclined  about  30°  and  the  motion  of  the  bars  gradually 


FIG.  38.— Wilkinson  Front  Inclined  Overfeed  Stoker. 

moves  the  coal  downwards,  and  deposits  the  ashes  and  clinkers 
on  the  clinker  grates,  from  which  they  are  finally  pushed  into 
the  ash  pit.  The  grate  bars  are  cast  hollow  with  nearly 
horizontal  openings  leading  from  the  interior  of  the  bars 
through  the  risers  of  the  steps  that  form  the  upper  surface 
of  the  bars.  Each  grate  bar  is  given  a  to-and-from  motion  in 
a  horizontal  direction  by  the  rock  shaft  and  links,  the  ends 
of  the  bars  being  supported  by,  and  sliding  on,  the  hollow 
cast-iron  bearing  bars.  Practically  all  the  air  from  the  com- 
bustion of  the  coal  is  drawn  into  the  upper  ends  of  the  hollow 


62  MECHANICAL  STOKERS 

grate  bars  by  the  steam  jets,  and  forced  into  the  fire  from  the 
openings  in  the  tops  of  the  bars.  A  sprung  or  suspended  arch 
is  used  with  this  stoker  at  the  front. 


UNDERFEED  STOKERS  (Multiple  Retort) 

Riley  Stoker.— The  Riley  stoker  (Fig.  39)  is  of  the  multiple 
inclined  retort  class,  using  forced  draft  for  its  operation. 
Essentially,  it  is  made  up  of  individual  retorts  each  having 
horizontal  plungers  for  feeding  the  coal  (about  12  to  15  pounds 
of  coal  being  fed  per  stroke),  reciprocating  retorts,  inclined 
about  20  degrees,  moving  overfeed  grates,  and  adjustable 


FIG.  39. — Riley  Multiple  Retort  Underfeed  Stoker. 

apron  refuse  supporting  plates,  each  retort  weighing  about 
4,000  pounds. 

The  fuel-feeding  mechanism  is  made  up  of  cylindrical 
horizontal  rams,  9"  in  diameter,  set  about  19"  centers.  These 
rams  or  plungers  are  connected  to  the  crank  shaft  by  connect- 
ing rods.  All  ram  boxes  are  set  in  line  and  bolted  to  angles. 
The  crank  shaft  bearing  brackets  are  bolted  to  the  ram  boxes 
and  tie  them  together.  Crank  shafts  are  made  in  sections, 
each  section  being  driven  by  a  gear  box. 

Eeduction  gearing  is  used  to  drive  the  main  crank  shaft. 
The  main  worm  on  the  crank  shaft  is  driven  by  a  worm 
mounted  on  a  supported  shaft.  This  shaft  carries  a  cut  worm 


MECHANICAL  STOKERS 


63 


wheel  which  is  driven  by  a  hardened  steel  worm  mounted  on 
the  speed  shaft  of  the  stoker.  The  reduction  through  the 
gears,  that  is,  the  ratio  of  speed  shaft  to  crank  shaft,  is  330  to 
1.  The  speed  shaft  drive  may  be  taken  from  any  convenient 
source,  usually  from  a  line  shafting  running  about  400  R.  P.  M. 

A  y2"  shearing  pin  is  used  in  the  connecting  rod  to  provide 
against  breakage  of  parts,  should  the  rams  become  obstructed 
in  any  position  of  its  stroke. 

The  retorts  of  the  underfeed  section  are  inclined  about  20°, 
and  the  reciprocating  movement  is  obtained  by  side  rods 
driven  by  extension  of  the  main  plunger  wrist  pins.  The  side 


FIG.  40.— Rear  View  Riley  Multiple  Retort  Underfeed  Stoker. 


rod  bolts  withdraw  the  side  bars  to  the  same  position  011  each 
return  stroke.  The  travel  towards  the  bridge  wall  is  varied 
by  changing  the  amount  of  lost  motion  between  the  wrist  pins 
and  side  rod  by  adjusting  blocks.  The  retorts  move  a  maxi- 
mum of  V. 

On  top  of  the  retort  sides  are  placed  tuyere  blocks  with 
openings  for  air  admission  to  the  fuel  bed.  These  tuyeres  inter- 
lock and  are  bolted  together. 

The  overfeed  grates  (Fig.  40)  extend  the  full  width  of  the 
stoker,  and  are  made  up  of  unit  retort  width.  They  reciprocate 
back  and  forth  practically  the  same  as  the  reciprocating 
retorts. 

The  refuse  supporting  plates  are  hinged  together  by  chains 
to  form  an  apron,  which  hangs  down  over  the  ends  of  adjust- 


64  MECHANICAL  STOKERS 

able  racks.  These  racks  are  set  by  hand  to  the  size  of  the 
opening. 

Air  is  admitted  under  the  front  of  the  stoker  and  forced 
through  the  tuyeres  to  the  fuel  bed.  A  damper  is  placed  between 
the  main  air  chamber  underneath  the  front  of  the  stoker  and 
the  overfeed  reciprocating  grates  so  that  air  can  be  regulated 
for  this  part  of  the  stoker. 

Jones  A-C-Stoker. — The  Jones  A-C-Stoker  is  a  gradual 
development  from  the  original  single  retort  Jones-Stoker.  The 
A-C  stoker  is  of  the  inclined,  multiple  retort  class  using  forced 
draft  for  its  operation.  The  stoker  is  essentially  made  up  of 
horizontal  rams — inclined  retorts — stationary  overfeed  sections, 
and  single  dumping  grates  (Fig.  41). 

The  fuel  is  fed  to  the  furnace  from  the  hopper  located 
directly  in  front  of  the  stoker  by  horizontal  steam  operated 
rams.  In  the  underfeed  section  the  volatile  gases  are  distilled 
and  the  fuel  bed  progresses  onto  dead  plates  at  the  rear  of 
the  retort  and  then  onto  the  dump  grates. 

The  fuel  feeding  mechanism  consist  of  a  steam  cylinder 
for  each  retort.  A  ram  is  connected  to  the  piston  of  this 
cylinder.  "When  steam  is  admitted  into  the  cylinder  through 
an  automatic  control  valve — the  ram  is  drawn  outward  and  a 
charge  of  coal  falls  into  the  ram  case.  The  automatic  con- 
trol valve  reverses  the  stoke  and  the  charge  is  forced  into  the 
retort. 

A  rod  is  connected  to  the  ram  and  travels  through  the  front 
end  of  the  retort  below  the  ram  with  each  operation  of  the 
steam  piston.  On  this  rod  inside  the  retort  are  bolted  two 
pusher  blocks  so  that  the  coal  is  carried  upward  and  rear- 
ward with  each  stroke  of  the  ram. 

The  dump  plates  are  made  in  sections  and  are  hinged  to  a 
shaft  located  at  the  rear  of  the  overfeed  sections.  The  dump 
plates  projects  towards  the  bridge  wall,  and  are  balanced  and 
operated  from  the  side  of  the  furnace. 

The  standard  tuyeres  are  fitted  to  the  retort  sides — and  so 
formed  so  as  to  make  a  continuation  of  the  retort,  rounding 
off  on  top.  Special  side  tuyeres  are  used  reaching  quite  high 
above  the  side  wall — air  is  circulated  through  these  tuyeres — 
and  they  are  so  designed  to  keep  the  fuel  bed  away  from  the 


MECHANICAL  STOKERS  65 

brickwork  of  the  side  wall  and  in  this  way  prevent  clinker 
formation  and  bridging  over  the  retort. 

A  distinct  feature  of  this  stoker  is  the  control  valve.  It 
is  installed  in  the  steam  line  to  each  stoker  cylinder  and  has 
eight  rates  of  operation.  The  length  of  the  stroke  of  the  pusher 


FIG.  41.— Jones'  "  A-C  "  Multiple  Retort  Underfeed  Stoker. 

blocks  can  be  adjusted  by  changing  a  pin  on  the  connecting 
bars  below  the  rams. 

Air  is  admitted  to  an  air  chamber  below  the  retorts  from 
the  main  air  duct ;  from  this  chamber  the  air  goes  through  the 
main  tuyeres,  the  side  tuyeres  and  the  overfeed  section.  Air 
distribution  boxes  are  used  at  the  front  above  the  mouth  of 
the  retort. 


66  MECHANICAL  STOKERS 

Taylor  Stoker.— The  Taylor  stoker  (Fig.  42)  was  the  first 
of  the  multiple  retort  class  using  forced  draft  for  its  opera- 
tion. 

The  stoker  is  made  up  of  two  horizontal  plungers  or  rams, 
one  above  the  other,  and  connected  together  by  links  for  feed- 
ing the  coal;  stationary  retorts  inclined  about  22°;  an  oscillat- 
ing overfeed  section;  and  a  single  leaf  dumping  grate,  each 
single  retort  weighing  about  5,000  pounds. 

The  coal  from  the  hopper  discharges  into  a  cylindrical 
chamber  in  which  the  9"  diameter  upper  ram  works.  This 
ram  is  operated  by  a  crank  shaft  at  right  angles  which  is 
driven  by  a  worm  and  gear.  The  distance  from  center  to 


FIG.  42.— Type  "  A  A  "  Taylor  Multiple  Retort  Underfeed  Stoker. 

center  of  each  ram  is  20%",  this  being  a  unit  retort  width.  A 
single  speed  shaft  drives  the  series  of  worms  and  gears  enclosed 
in  a  gear  box.  The  ratio  of  speed  shaft  to  the  main  crank 
shaft  is  352  to  1.  The  lower  horizontal  plunger  or  ram  is  con- 
nected to  the  upper  one  by  means  of  links,  and  its  lost  motion 
is  adjustable. 

Bearing  brackets  for  the  main  crank  shafts  are  bolted  to 
the  ram  boxes  and  serve  to  tie  them  rigidly  together.  These 
bearings  are  babbitted  and  provided  for  lubrication.  This 
shaft  operates  clockwise  (looking  from  the  right  side  of  the 
stoker  at  the  end  of  the  shaft),  the  shaft  operating  from  one 
revolution  in  45  seconds  to  one  revolution  in  a  minute  and  a 
half. 


MECHANICAL  STOKERS 


67 


In  order  to  provide  against  damage  due  to  any  foreign 
substances  that  might  block  the  action  of  the  rams,  a  shearing 
pin  is  provided  in  the  speed  shaft  and  the  parts  so  designed 
that  stresses  are  transmitted  to  this  part  of  the  mechanism. 

In  the  standard  stoker,  17  tuyere  blocks  are  used,  set 
immediately  on  top  of  the  tuyere  boxes.  Each  tuyere  block 
hooks  into  recesses  and  interlock  with  each  other,  thus  hold- 
ing them  in  place. 

The  air  for  combustion  is  forced  from  the  wind  box  through 
the  tuyeres  to  the  fuel  bed  and  controlled  by  suitable  dampers. 


FIG.  43.— Rear  View  Taylor  Type  "  A  A  "  Multiple  Retort  Underfeed  Stoker. 

The  air  for  the  oscillating  overfeed  section  is  alsa  controlled 
by  dampers. 

The  dumping  grates  (Fig  43)  are  made  up  of  frames  to 
which  are  fitted  interchangeable  grate  bar  tops,  these  being 
of  the  unit  retort  width.  The  dumping  grate  is  operated  by 
means  of  levers  in  front  of  the  stoker. 

The  stokers  for  different  size  furnace  widths  are  built  up 
by  a  number  of  retorts.  For  example,  the  furnace  for  a  500- 
H.  P.  boiler  might  be  made  up  of  six  retorts. 

Different  lengths  from  the  front  wall  to  the  bridge  wall 
are  used.  In  some  cases,  the  dump  grates  are  operated  by  power 


68  MECHANICAL  STOKERS 

and  in  special  installations  a  clinker  grinder  is  used  for  ash 
disposal. 

Westinghouse  Stoker. — The  Westinghouse  stoker  (Fig.  44} 
is  a  multiple  retort  inclined  about  20°,  and  uses  forced  draft  for 
its  operation.  The  stoker  is  essentially  made  up  of  downward 
inclined  rams,  inclined  retorts,  a  reciprocating  overfeed  section 
and  double  dumping  grates. 

The  fuel  is  fed  to  the  furnace  from  the  hopper  proper  by 
downward  inclined  rams  equal  in  number  to  the  retorts  in 


FIG.  44. — Westinghouse  Multiple  Retort  Underfeed  Stoker. 

the  furnace.  In  the  underfeed  section,  the  volatile  gases  are 
distilled  and  the  fuel  bed  progresses  on  to  the  overfeed  section 
and  then  on  to  the  dumping  grates.  About  18  Ibs.  of  coal  is 
pushed  into  the  underfeed  section  with  each  stroke  of  the  ram. 

The  fuel-feeding  mechanism  consists  of  a  9"  plunger  or  ram 
operating  in  an  inclined  cylinder  forming  part  of  the  ram  box, 
these  rams  being  set  21"  centers.  These  rams  obtain  their 
motion  through  connecting  rods  to  cast  steel  crank  shafts 
3y2"  diameter,  these  being  made  up  of  units  and  connected 
together  with  bolts. 

Bearing  brackets  for  the  crank  shafts  are  bolted  to  the  ram 
boxes  and,  in  this  way,  tie  them  together.  These  bearings  in 


MECHANICAL  STOKERS  69 

the  brackets  are  babbitted  and  provided  with  grease  cups  for 
lubrication.  This  shaft  operates  from  one  revolution  in  45 
seconds  to  one  revolution  in  a  minute  and  a  half. 

The  reduction  worm  gearing  is  used  to  drive  the  crank 
shaft.  The  main  power  worm  gear  is  connected  by  bolts  to 
the  flanges  of  the  crank  shaft,  and  is  driven  by  a  worm 
mounted  on  a  cross  shaft  supported  in  the  gear  box.  This 
shaft,  in  turn,  is  driven  by  a  bronze  worm  gear  and  hardened 
steel  worm  fitted  to  the  speed  shaft.  One  gear  set  is  used  to 
drive  a  maximum  of  four  retorts.  A  hinged  cover  on  the  gear 
case  is  provided  to  make  inspection.  The  speed  reduction  in 
the  gear  box  is  352  to  1. 

A  protective  device,  consisting  of  a  3/32"  shearing  pin,  is 
provided  in  connection  with  the  speed  shaft  to  guard  against 
damage  of  stoker  parts,  should  foreign  substances  be  acci- 
dentally admitted  with  the  coal  and  block  the  movement  of  the 
rams. 

The  lower  ram  is  operated  by  means  of  connecting  rods, 
through  lost-motion  mechanism,  to  the  upper  ram.  It  recipro- 
cates along  the  bottom  of  the  retort,  and  in  the  same  plane 
as  the  main  ram. 

Above  the  upper  end  of  the  tuyere  boxes,  and  extending 
laterally  across  the  furnace,  are  air  distributing  boxes  which 
are  bolted  to  the  ram  boxes.  These  are  made  of  unit  retort 
width,  cast  hollow,  and  designed  to  transmit  part  of  the  load 
of  the  front  wall  to  the  tuyere  boxes. 

A  fuel  deflecting  plate  is  provided  in  the  retort  for  varying 
the  thickness  and  outline  of  the  fuel  bed. 

The  forward  and  rear  dumping  grates  are  made  up  of 
frames  to  which  are  fitted  interchangeable  corrugated  grate 
bar  tops,  and  in  this  way  a  passage  for  air  is  formed.  These 
are  made  of  the  unit  retort  width,  the  lower  grate  bar  top  being 
bolted  so  as  to  lock  the  other  tops  in  place.  The  forward  dump- 
ing grate  is  pivotally  connected  to  the  dump  grate  brackets, 
and  projects  towards  the  bridgewall,  and  the  rear  dumping 
grate  is  supported  in  a  similar  manner  and  projects  towards 
the  front  of  the  stoker.  Both  dump  grates  are  operated  from 
the  side  of  the  furnace. 

The  overfeed  grate  is  of  unit  (Fig.  45)  retort  width,  and  is 


70 


MECHANICAL  STOKERS 


operated  by  the  upper  ram  through  rods  and  lost-motion  con- 
nection. The  rods  return  the  overfeed  grate  to  the  same  posi- 
tion on  each  stroke.  The  travel  forward  is  adjusted  by  chang- 
ing the  amount  of  lost-motion  by  means  of  collars.  These  are 
placed  on  the  rods  from  the  front  of  the  stoker. 

The  furnace  is  sealed  from  the  stoker  front  to  the  rear  of 
tuyere  boxes  by  means  of  a  reinforced  concrete  floor.  Dampers 
are  provided  and  set  in  the  floor  to  control  the  air  from  the 
main  air  ducts.  Auxiliary  dampers  divide  the  wind  box  trans- 
versely. Doors  are  placed  in  the  front  for  access  to  the  wind 
box  and  for  air  admission  when  the  stokers  are  being  operated 
on  natural  draft. 


FIG.  45. — Rear  View  Westinghouse  Multiple  Retort  Underfeed  Stoker. 

The  tuyere  boxes  used  with  the  stokers  are  of  the  box 
girder  type.  Their  front  ends  are  bolted  to  the  ram  boxes  and 
rest  on  angles  supported  from  the  floor.  Their  rear  ends  are 
tied  together.  The  ribs  on  the  sides  of  the  tuyere  boxes  are 
used  for  supporting  the  retort  bottoms. 

The  tuyeres  are  of  corrugated  construction  and  hooked  into 
the  recesses  in  the  top  of  the  tuyere  boxes  and  interlock  with 
each  other,  thus  holding  them  in  place.  In  a  stoker  10  ft. 
from  the  front  wall  to  the  bridge  wall  of  the  double  dump 
grate  type,  17  tuyeres  are  used.  They  are  laid,  one  upon  the 
other,  and  the  entire  block  is  locked  by  means  of  the  upper 
tuyere. 

Air  admission  to  all  parts  of  the  furnace  is  controlled  from 
the  front  or  side  of  the  stoker.  Air  is  admitted  to  air  distrib- 
uting boxes  located  at  the  front  for  air  above  the  fuel  bed. 

The  air  through  the  underfeed  section  is  forced  from  the 


MECHANICAL  STOKERS  71 

wind  box  through  the  tuyeres  to  the  fuel  bed,  and  controlled 
bj  dampers.  Air  admission  to  the  overfeed  section  and  dump 
grates  is  also  controlled  by  dampers. 

Frederick  Stoker. — The  Frederick  Underfeed  Stoker  is  of 
the  general  inclined  multiple  retort  type,  having  horizontal 
rams;  stationary  underfeed  sections,  and  air  admitting  dump- 
ing grates. 

The  retorts  are  spaced  21  inches  apart  and  have  a  main 
feeding  ram  O1/^  inches  diameter,  feeding  approximately  20 
pounds  of  Eastern  fuel  per  stroke. 

The  underfeed  section  is  inclined  20  degrees  from  the  hori- 
zontal and  the  usual  extension  grate  common  to  this  type  of 
stoker  is  eliminated. 

The  secondary  rams  are  operated  by  suitable  connection 
to  the  main  ram  mechanism  and  the  stroke  of  this  ram  is 
controlled  by  means  of  a  shaft  located  above  the  retorts  in 
front  which  engages  or  disengages  the  dogs  fastened  to  the 
moving  rod ;  thus  increasing  the  stroke  of  the  secondary  ram  to 
a  maximum  of  6  inches  or  allowing  it  to  operate  at  normal  stroke. 

The  shearing  pin  arrangement  is  located  in  the  speed  shaft 
mechanism  as  is  common  with  other  stokers  of  this  type.  In 
order  to  obtain  two  sets  of  speeds  for  the  rams,  there  are  two 
sprockets  placed  on  the  speed  shaft  and  by  operating  these 
through  a  clutch  arrangement  one  or  the  other  ratio  of  speeds 
can  be  used. 

Kefractory  blocks  are  used  in  the  front  wail  and  bridge 
wall  immediately  over  the  stoker  parts. 

UNDERFEED  STOKERS  (Single  Retort) 

Jones  Stoker. — The  Jones  stoker  (Figs.  46  and  47)  is  of  the 
single  retort  class  essentially  consisting  of  one  horizontal  retort 
with  plates  on  the  side  for  supporting  a  fuel  bed  as  it  is  pushed 
out  of  the  retort.  Forced  draft  is  used  with  this  equipment. 

The  fuel  feeding  mechanism  consists  of  a  small  hopper  bolted 
onto  the  ram  case,  which,  in  turn,  is  bolted  to  a  steam  cylinder, 
all  of  this  mechanism  being  outside  of  the  furnace. 

Steam  is  admitted  behind  the  piston  which  forces  the  piston 
and  ram  forward  carrying  a  portion  of  the  coal  in  the  hopper 


72 


MECHANICAL  STOKERS 


to  the  retort.    Pusher  rods  are  provided  in  the  retort  to  carry 
a  portion  of  the  coal  forward  in  the  retort. 


FIG.    4b'. — Sectional  Front  View  Jones  Single  Retort  Underfeed  Stoker 
with  Side  Plates. 

The  retort  consists  of  a  fuel  magazine  with  tuyere  blocks 
attached  to  the  top  of  the  retort.  These  tuyere  blocks  are 
made  in  unit  sections  and  are  fastened  to  the  retort  by  one 
single  rod. 


FIG.  47. — Jones  Single  Retort  Side-dump  Underfeed  Stoker. 

Air  from  the  blower  is  forced  into  a  sealed  ash  pit  from 
which  the  air  passes  through  the  hollow  tuyeres  to  the  fuel  bed. 

The  dead  plates  on  the  side  are  ribbed  and  the  air  circulates 
under  them  and  through  the  tuyere  blocks.  No  air  is  admitted 
through  them. 


MECHANICAL  STOKERS 


73 


The  operation  of  the  steam  rams  are  controlled  by  means 
of  automatic  valves,  each  ram  being  adjusted  independently 
of  the  other.  The  rate  of  feed  can  also  be  adjusted  within  a 
certain  range  of  operation. 

Doors  are  placed  in  the  stoker  front  for  use  in  cleaning 
the  fires  as  the  ash  is  separated  from  the  incandescent  fuel  bed 
and  pulled  out  of  these  doors. 

Moloch  Stoker. — This  stoker  is  of  the  single  retort  class 
using  forced  draft.  A  retort  runs  lengthwise  of  the  furnace 
into  which  the  fuel  is  pushed  by  a  steam  ram.  Above  the  retorts 


FIG.  48. — Moloch  Single  Retort  Underfeed  Stoker. 

are  tuyeres  to  supply  the  air  to  the  fuel  bed,  and  on  either  side 
is  a  rotary  grinder  disposing  of  the  refuse.  The  fresh  fuel  is 
fed  underneath  the  zone  of  combustion,  there  being  no  moving 
or  stationary  grates  to  provide  for  overfeed  burning. 

Fig.  48  is  a  perspective  of  a  two-retort  equipment.  The 
retorts  are  inclosed  by  cast-iron  air  boxes  with  the  air  supply 
for  each  controlled  independently  by  an  air  gate  operated  from 
the  front  of  the  boiler.  Each  stoker  unit  is  supported  at  the 
front  and  at  the  bridgewall. 

The  retort  has  a  horizontal  top  on  which  are  mounted  tuyere 
blocks  to  provide  air  under  pressure,  the  fuel  being  fed  to 


74 


MECHANICAL  STOKERS 


the  retort  by  a  steam  operated  ram  supplied  from  the  hopper 
attached  to  the  ram  cage.  The  operation  of  the  ram  is  inter- 
mittent and  automatically  regulated,  each  charge  delivering 
coal  to  the  retort.  The  tuyere  openings  supply  the  air  for  com- 
bustion below  the  fire  and  above  the  fresh  fuel.  The  heat  of 
the  fire  above  drives  off  the  volatile  matter,  and  these  gases 


FIG.  49.— Type  "  E  "  Single  Retort  Underfeed  Stoker. 

mix  with  the  air  supply  passing  through  the  zone  of  com- 
bustion. The  coke  passes  upward  and  out  of  the  retort  and 
above  the  grinders,  the  refuse  passing  over  the  tuyere  blocks 
to  the  rotary  grinders. 

Type  "E"  Stoker. — The  Type  "E"  stoker  is  of  the  single 
reton  class  and  uses  forced  draft. 

The  coal  is  fed  into  a  hopper  on  the  outside  of  the  furnace. 
From  this  hopper  (Fig.  49)  the  fuel  is  delivered  by  gravity  to 
the  front  end  of  the  bottom  of  the  retort,  and  is  then  pushed 


MECHANICAL  STOKERS 


75 


into  the  furnace  by  a  steam-driven  feeder  block  until  the  retort, 
which  runs  the  length  of  the  furnace,  is  filled. 


FIG.  60.— Cross  Section  Type  "  E  "  Single  Retort  Underfeed  Stoker. 

As  the  coal  rises  in  the  retort,  it  is  flooded  on  the  fire-bars 
(Fig.  50)  by  the  movement  of  auxiliary  pushers.  The  grate  bars 
are  arranged  alternately  moving  and  fixed  and  reciprocate  with 
the  movement  of  the  pusher/  spreading  the  coked  fuel  toward 


76 


MECHANICAL  STOKERS 


the  side  or  dump  plates.  The  movement  of  the  fire-bars  also 
conveys  the  clinker  and  ashes  to  the  dump  trays. 

The  bars  are  slightly  inclined,  but  not  enough  to  cause  a 
gravity  travel  of  the  coal.  The  fire  is  kept  moving  by  the 
mechanical  movements  of  the  bars,  obtained  by  means  of  two 
helices  and  nuts  placed  outside  the  furnace.  These  cause  the 
moving  bars  to  rock  to  and  fro. 

Each  stoker  has  an  individual  drive,  but  by  means  of  auto- 
matic control  all  are  run  at  a  uniform  speed.  A  stoker  may 


FIG.  51. — Detroit  Single  Retort  Underfeed  Stoker. 

be  cut  in  or  out  of  service  by  throttling  the  driving  engine 
and  shutting  off  the  air. 

Detroit  Underfeed  Stoker. — This  stoker  (Fig.  51)  is  of  the 
single  retort  type  and  uses  forced  draft  for  its  operation.  It 
consists  of  a  horizontal  retort  with  side  grates  for  supporting 
the  fuel  bed  as  it  is  pushed  out  of  the  retort,  and  dumping 
grates  on  each  side  for  cleaning  purposes.  It  has  mechanically 
operated  plungers,  or  rams,  connected  through  a  reduction  gear 
to  a  shaft  located  in  front  of  the  stoker.  All  the  gears,  worms 


MECHANICAL  STOKERS 


77 


and  moving  parts  are  enclosed  in  a  gear  box.  This  gear  box 
is  similar  in  construction  to  the  gear  box  used  on  the  multiple 
inclined  underfeed  stokers. 

The  stoker  consists  of  a  coal  hopper  in  front,  and  the  retort 
proper,  extending  into  the  furnace.  Connected  to  the  ram  or 
plunger  is  a  pusher  block  located  in  the  bottom  of  the  retort 
and  is  used  to  feed  the  coal  the  entire  length  of  the  fuel  bed. 


FIG.  52.— Rear  View  Detroit  Single  Retort  Underfeed  Stoker. 

Tuyere  blocks  for  distribution  of  the  air  to  the  fuel  bed  are 
made  of  unit  sections,  and  are  fitted  to  both  sides  of  the  retort. 

Air  from  a  blower  is  forced  into  sealed  compartments  from 
which  it  is  directed  by  the  tuyeres  to  the  fuel  bed. 

The  grates  on  each  side  (Fig.  52)  of  the  retort  are  ribbed, 
and  are  made  to  extend  from  the  tuyere  blocks  to  the  dumping 
grates,  located  at  the  sides.  The  dumping  grates  are  solid, 
without  air  admission,  and  are  operated  from  the  front  of  the 
stoker. 

The  operation  of  the  mechanical  rams  is  taken  from  the 
crank  shaft  and  controlled  by  lengthening  or  shortening  the 


78 


MECHANICAL  STOKERS 


stroke  of  the  connecting  rod,  each  rod  being  adjusted  inde- 
pendently. The  rate  of  feed  can  be  adjusted  within  a  certain 
range  of  operation. 

Doors  are  placed  in  the  stoker  front  for  use  in  cleaning 
fires,  these  doors  are  located  immediately  in  front  of  the  dump 
grates  and  a  view  of  the  fires  can  be  had  at  all  times  from  the 
front.  Ash  pit  doors  are  also  located  in  the  stoker  front,  so 
that  ashes  can  be  cleaned  out  from  the  front  of  the  stoker. 

Roach  Stoker. — This  stoker  is  of  the  single  retort  type — 
with  side  grates  and  dump  grates  next  to  the  furnace  wall. 


FIG.  53. — Roach  Single  Retort  Underfeed  Stoker. 

Coal  is  conveyed  from  a  hopper  located  in  front  by  the  action 
of  a  plunger  operated  from  a  steam  cylinder.  This  plunger 
pushes  the  coal  into  the  retort  where  it  is  distributed  to  the 
inclined  side  grates,  which  have  a  reciprocating  motion.  The 
fuel  bed  progresses  over  the  side  grates  to  the  dump  grates 
which  are  made  up  in  sections.  The  ash  and  refuse  is  dumped 
into  the  ash  pit,  and  removed  from  the  front  of  the  stoker  or 
from  a  tunnel  below. 

The  motion  of  the  side  grates  can  be  adjusted  so  that  a 
certain  movement  of  the  bars  can  be  obtained. 

The  air  distribution  is  controlled  from  a  central  air  cham- 
ber; from  this  chamber  air  is  directed  to  the  ends  of  the 


MECHANICAL  STOKERS  79 

grate  bars  next  to  the  dump  grates  and  then  travels  through 
the  bars  and  enters  the  furnace  or  fuel  bed  through  the  head 
of  the  bars.  Air  is  also  directed  to  a  chamber  immediately 
under  the  retort,  being  controlled  with  a  valve,  shutting  this 
chamber  off  from  the  main  wind  box.  The  air  taken  from  this 
so-called  low-pressure  air  chamber  and  travels  upwards  and 
passes  through  the  interstices  between  the  grate  bars. 

An  ignition  arch  is  not  necessary  with  this  stoker  unless 
the  boiler  and  stoker  application  requires  it.  The  stoker  op- 
erates on  forced  draft  the  same  as  other  stokers  of  this  class. 


CHAPTER   III 

COAL  AND  COAL-PRODUCING  FIELDS  OF  THE 
UNITED  STATES 

COAL— DEFINITIONS 

General. — According  to  the  general  meaning,  coal  is  a  solid 
fuel  and  it  is  something  which  enters  combustion  and  produces 
heat. 

Ash-  and  Moisture-Free  Coal  is  that  part  of  the  fuel,  minus 
ash  and  moisture,  as  neither  of  these  take  part  in  the  combus- 
tion processes,  nor  do  they  develop  heat. 

Clean  Coal. — Properly  prepared  lump  coal;  for  example, 
consisting  of  fuel  in  which  there  is  no  visible  ash,  or,  in  other 
words,  consisting  of  clean,  black  pieces  accompanied  by  no 
slate  or  other  dirt,  the  inference  being  that  there  are  no  in- 
visible impurities  with  the  coal. 

Dirty  Coal. — A  fuel  mixture  containing  a  large  amount  of 
foreign  matter,  such  as  slate,  fire  clay,  rock,  etc. 

Size  of  Coal. — This  term  is  used  to  denote  the  sizes  .of  coal 
pieces.  An  example  of  this  application  would  be  "1^4" 
screenings." 

Kind  of  Coal. — This  expression  is  used  in  the  classification 
of  coals,  such  as  the  following  examples : 

Anthracite,  Semi-Anthracite,  Bituminous,  Semi-Bituminous, 
Lignite,  Coking  Coal,  Gas  Coal,  Dry  Coal,  Moist  Coal,  etc. 

Grade  of  Coal. — As  an  example,  anthracite  or  bituminous 
are  not  grades  of  coal,  but  kinds.  The  application  of  the  term 
grade  is  shown  by  the  following  examples:  Mine  run,  lump, 
egg,  nut,  washed  coal,  washed  slack,  washed  screenings,  etc. 

Caking  Coal. — Some  coals  tend  to  form  a  solid  mass  when 
heated  in  a  retort  or  furnace.  This  characteristic  makes  the 
coal  difficult  to  burn  by  any  means  which  does  not  provide 
an  agitation  to  break  up  the  cake  during  its  processes  of  f  orma- 

80 


COAL  AND  COAL-PRODUCING  FIELDS  81 

tion.  This  action  is  due  to  the  peculiarity  of  the  volatile  matter 
but  after  the  volatile  has  been  driven  off  and  the  caked  mass 
broken  up,  there  is  no  further  tendency  for  the  fuel  to  again 
cake  in  this  manner.  Such  coal  will  be  known  as  caking  coal. 
Coals  which  are  most  suitable  for  making  coke  possess  this 
property. 

Clinker. — Clinker  is  ash  which  has  been  fused.  These  coals 
containing  ash  having  a  low  fusing  temperature  are  commonly 
referred  to  as  clinkering  coals  because  when  they  are  burned 
in  the  ordinary  furnace,  a  considerable  percentage  of  the  ash  is 
reduced  to  clinker.  Those  coals  whose  ash  has  a  high  fusing 
temperature  are  commonly  referred  to  as  non-clinkering.  It  is 
apparent,  however,  that  if  fuel  is  being  burned  economically 
a  comparatively  small  amount  of  excess  air  is  being  introduced 
into  the  furnace  and  high  combustion  temperatures  must  result. 
The  actual  temperature  of  a  particle  of  coal  under  such  condi- 
tions is  considerably  in  excess  of  the  fusing  temperature  of 
ash  even  in  the  so-called  non-clinkering  coal.  It  therefore  fol- 
lows that  if  coal  is  being  burned  economically,  a  large  per- 
centage of  the  ash  will  be  fused.  If  the  method  of  burning  coal 
is  such  that  large  masses  of  incandescent  fuel  in  the  latter 
stages  of  combustion  are  agitated  there  is  a  tendency  to  produce 
large  clinkers,  whereas  if  the  fuel  is  not  disturbed,  smaller 
masses  of  clinker  will  result.  Clinker  becomes  troublesome  only 
when  it  adheres  to  furnace  brickwork  or  accumulates  in  such 
quantities  that  it  interferes  with  the  operation  of  the  furnace, 
and  where  clinker  troubles  are  hereafter  referred  to  it  will  be 
understood  to  refer  to  these  large  accumulations.  Coal  having 
ash  of  low  fusing  temperature  will  not  necessarily  cause  clinker 
troubles.  If  the  percentage  of  ash  be  low,  the  stoking  mechan- 
ism will  usually  be  able  to  accumulate  and  discharge  the  clinker 
from  the. furnace  without  interfering  with  operation.  On  the 
other  hand,  a  coal  comparatively  high  in  ash  which  has  a  high 
fusing  temperature  can  usually  be  handled  without  trouble  be- 
cause all  of  the  ash  will  not  be  reduced  to  clinker  and  the 
accumulations  will  not  be  large  enough  to  interfere  with  opera- 
tion. 

Coking  Coal. — Some  kinds  of  coal,  when  heated,  give  off 
the  volatile  constituents  at  relatively  low  temperatures,  and 


82 


MECHANICAL  STOKERS 


leave  the  carbon  in  a  compact  solid  mass.     This  is  generally 
known  as  coking  coal. 


USE   OF  COALS 

Coal  is  used  in  the  United  States  to  a  greater  extent  than 
any  other  one  material,  and  the  supply  is^of  general  importance 
to  the  industries.  Coal  is  essential  to  the  small  enterprise  as 
well  as  the  public  utility  that  develops  and  sells  power 
(Fig.  54). 


55 


Wafer  Purification  Labor 
Water  Purification  Supplies 

Mainf.  Boiler  Planf  Bldg.  and  Grounds    \ 
Mainf.  Wafer  Purification  Equipment-' 

FIG.  54. — "Relation  of  Coal  Expenses  to  Other  Operating  Expenses  of  the 
Generating  Plant  in  Public  Utility  Operation." 

For  the  year  of  1915  the  Geological  Survey  obtained  data 
which  show  the  relative  importance  of  the  ways  in  which 
bituminous  coal  is  used.  The  percentage  of  the  total  consump- 
tion in  1915  was  as  follows : 


COAL  AND  COAL-PRODUCING  FIELDS 


83 


Exports 4% 

Steamship  bunkers  at  tidewater.  2% 
Used  at  mines  for  steam  and  heat.  2% 
Manufacture  of  coal  gas 1% 


Industrial  steam  trade 33% 

Railroad  fuel 28% 

Domestic  and  small  steam  trade.  16% 
Manufacture  of  beehive  coke. .  .  9% 
Manufacture  of  by-product  coke  4% 

Other  statistics  by  manufacturers  were  computed  by  the 
Bureau  of  the  Census  for  1914.  Bituminous  coal  was  then,  ac- 
cording to  these  figures,  used  most  largely  in  the  manufacture 
of  the  following  articles  (the  figures  represent  net  tons) : 


Coke 50,457,000 

Steel    works    and    rolling 

mills 20,343,000 

Brick,  tile  and  other  clay 

products 8,566,000 

Cement 6,731,000 

Chemicals 2,667,000 

Glass 2,252,000 

Petroleum  refining 2,045,000 

Blast  furnace 1,892,000 

Flour-mill    and    grist-mill 

products.  .  .. 1,809,000 

Woolen  and  worsted  goods.  1,544,000 
Oil,  cottonseed  and  cake. .  .  1,232,000 
Leather,  tanned,  curried 

and  finished 1,124,000 

Zinc  smelting  and  refining. .  1,066,000 

Rubber  goods 919,000 

Paper  and  wood  pulp 6,268,000 

Gas,  illuminating  and  heat- 
ing   6,078,000 

Car  and  general  shop  con- 
struction and  repair  by 

steam  railways 5,486,000 

Distilled  liquors 909,000 

Dyeing  and  finishing  tex- 
tiles       896,000 

Lumber  and  timber  prod- 
ucts       885,000 


Sugar  refining 875,000 

Copper  smelting  and  refin- 
ing       812,000 

Electrical  machinery,  appa- 
ratus, etc 769,000 

Furniture 751,000 

Salt 714,000 

Steam-railway  cars,  private 

bunders 698,000 

Beet  sugar 682,000 

Cotton  goods 3,579,000 

Ice,  manufactured 3,386,000 

Foundry  and  machine-shop 

products 2,913,000 

Slaughtering  and  meat 

packing 2,786,000 

Malt  liquors 2,749,000 

Lime 677,000 

Paving  materials 665,000 

Glucose  and  starch  pottery .      577,000 
Agricultural  implements. . .      555,000 

Wire 523,000 

Soap 515,000 

Marble  and  stone  work 485,000 

Hosiery  and  knit  goods ....      484,000 

Automobiles 464,000 

Planing-mill  products,  not 
including  mills  connected 

with  saw  mills 457,000 

Fertilizers 433,000 


WORLD'S   RESERVE 

On  account  of  the  many  unknown  factors,  an  estimate  on 
the  world's  coal  supply  is  more  or  less  speculative.  According 
to  an  inquiry  made  in  1913,  the  coal  reserves  of  the  world 
available  in  the  principal  coal-producing  countries,  stand  as 
follows : 


84 


MECHANICAL  STOKERS 


Short  Tons 

United  States,  including  Alaska 4,231,352,000,000 

Canada 1,360,535,000,000 

China 1,097,436,000,000 

Germany 466,665,000,000 

Great  Britain  and  Ireland 208,922,000,000 

Siberia 191,667,000,000 

Australia 182,510,000,000 

India 87,083,000,000 

Russia  in  Europe 66,255,000,000 

Union  of  South  Africa 61,949,000,000 

Austria 59,387,000,000 

Colombia 29,762,000,000 

Indo-China 22,048,000,000 

France 19,382,000,000 

Other  countries 69,369,500,000 


8,154,322,500,000 


WORLD'S  PRODUCTION 

The  latest  statistics  available  with  a  degree  of  completeness 
are  for  the  year  1914.  In  that  year,  in  a  world  production  in 
the  neighborhood  of  1,345,000,000  net  tons,  the  United  States 
contributed  38  per  cent,  Great  Britain  22  per  cent,  and  Ger- 
many 20  per  cent. 

In  the  year  1913,  for  which  estimates  are  more  complete, 
the  production  of  all  kinds  of  coal  by  the  more  important 
countries  was  approximately  as  follows: 


Net  Tons 

United  States 569,000,000 

Great  Britain 321,000,000 

Germany 305,000,000 

Austria-Hungary 59,000,000 


Net  Tons 

France 45,000,000 

Russia 35,000,000 

Belgium 25,000,000 

Japan 23,000,000 


COAL  PRODUCTION  IN  THE  UNITED  STATES 

In  1916,  the  United  States  produced  over  a  half  million 
tons  of  bituminous  coal,  and,  as  the  demand  for  coal  is  now 
unprecedented,  there  is  no  question  but  what  the  fields  will 
produce  more  than  ever  before.  The  coal  produced  in  the 
United  States  in  1807  (the  date  of  the  earliest  record)  to  the 
end  of  1915  is  shown  in  the  following  table: 


COAL  AND  COAL-PRODUCING  FIELDS 


85 


Year 

Pennsylvania 
Anthracite 

Bituminous 

Total 

1807-1820 

12,000 

3,000 

15,000 

1821 

1,322 

1,322 

1822 

4,583 

54,000 

58,583 

1823 

8,563 

60,000 

68,563 

1824 

13,685 

67,040 

80,725 

1825 

42,988 

75,000 

117,988 

1826 

59,194 

88,720 

149,914 

1827 

78,151 

94,000 

172,151 

1828 

95,500 

100,408 

195,908 

1829 

138,086 

102,000 

240,086 

1830 

215,272 

104,800 

320,072 

1831 

217,842 

120,100 

337,942 

1832 

447,550 

146,500 

594,050 

1833 

600,907 

133,750 

734,657 

1834 

464,015 

136,500 

600,515 

1835 

690,854 

134,000 

824,854 

1836 

842,832 

142,000 

984,832 

1837 

1,071,151 

182,500 

1,253,651 

1838 

910,075 

445,452 

1,355,527 

1839 

1,008,322 

552,038 

1,560,360 

1840 

967,108 

1,102,931 

2,070,039 

1841 

1,182,441 

1,108,700 

2,291,141 

1842 

1,365,563 

1,244,494 

2,610,057 

1843 

1,556,753 

1,504,121 

3,060,874 

1844 

2,009,207 

1,672,045 

3,681,252 

1845 

2,480,032 

1,829,872 

4,309,904 

1846 

2,887,815 

1,977,707 

4,865,522 

1847 

3,551,005 

1,735,062 

5,286,067 

1848 

3,805,942 

1,968,032 

5,773,974 

1849 

3,995,334 

2,453,497 

6,448,831 

1850 

4,138,164 

2,880,017 

7,018,181 

1851 

5,481,065 

3,253,460 

8,734,525 

1852 

6,151,957 

3,664,707 

9,816,664 

1853 

6,400,426 

4,169,862 

10,570,288 

1854 

7,394,875 

4,582,227 

11,977,102 

18-55 

8,141,754 

4,784,919 

12,926,673 

86 


MECHANICAL  STOKERS 


Year 

Pennsylvania 
Anthracite 

Bituminous 

Total 

1856 

8,534,779 

5,012,146 

13,546,925 

1857 

8,186,567 

5,153,622 

13,340,189 

1858 

8,426,102 

5,548,376 

13,974,478 

1859 

9,619,771 

6,013,404 

15,633,175 

1860 

8,115,842 

6,494,200 

14,610,042 

1861 

9,799,654 

6,688,358 

16,488,012 

1862 

9,695,110 

7,790,725 

17,485,835 

1863 

11,785,320 

9,533,742 

21,319,062 

1864 

12,538,649 

11,066,474 

23,605,123 

1865 

11,891,746 

11,900,427 

23,792,173 

1866 

15,651,183 

13,352,400 

29,003,583 

1867 

16,002,109 

14,722,313 

30,724,422 

1868 

17,003,405 

15,858,555 

32,861,960 

1869 

17,083,134 

15,821,226 

32,904,360 

1870 

15,664,275 

17,371,305 

33,035,580 

1871 

19,342,057 

27,543,023 

46,885,080 

1872 

24,233,166 

27,220,233 

51,453,399 

1873 

26,152,837 

31,449,643 

57,602,480 

1874 

24,818,790 

27,787,130 

52,605,920 

1875 

22,485,766 

29,862,554 

52,348,320 

1876 

22,793,245 

30,486,755 

53,280,000 

1877. 

25,660,316 

34,841,444 

60,501,760 

1878 

21,689,682 

36,245,918 

57,935,600 

1879 

30,207,793 

37,898,006 

68,105,799 

1880 

28,649,812 

42,831,758 

71,481,570 

1881 

31,920,018 

53,961,012 

85,881,030 

1882 

35,121,256 

68,429,933 

103,551,189 

1883 

38,456,845 

77,250,680 

115,707,525 

1884 

37,156,847 

82,998,704 

120,155,551 

1885 

38,335,974 

72,824,321 

111,160,295 

1886 

39,035,446 

74,644,981 

113,680,427 

1887 

42,088,197 

88,562,314 

130,650,511 

1888 

46,619,564  ' 

102,040,093 

148,659,657 

1889 

45,546,970 

95,682,543 

141,229,513 

1890 

46,468,641 

111,302,322 

157,770,963 

COAL  AND  COAL-PRODUCING  FIELDS 


87 


Year 

Pennsylvania 
Anthracite 

Bituminous 

Total 

1891 

50,665,431 

117,901,238 

168,566,669 

1892 

52,472,504 

126,856,567 

179,329,071 

1893 

53,967,543 

128,385,231 

182,352,774 

1894 

51,921,121 

118,820,405 

170,741,526 

1895 

57,999,337 

135,118,193 

193,117,530 

1896 

54,346,081 

137,640,276 

191,986,357 

1897 

52,611,680 

147,617,519 

200,229,199 

1898 

53,382,644 

166,593,623 

219,976,267 

1899 

60,418,005 

193,323,187 

253,741,192 

1900 

57,367,915 

212,316,112 

269,684,027 

1901 

67,471,667 

225,828,149 

293,299,816 

1902 

41,373,595 

260,216,844 

301,590,439 

1903 

74,607,068 

282,749,348 

357,356,416 

1904 

73,156,709 

278,659,689 

351,816,398 

1905 

77,659,850 

315,062,785 

392,722,635 

1906 

71,282,411 

342,874,867 

414,157,278 

1907 

85,604,312 

394,759,112 

480,363,424 

1908 

83,268,754 

332,573,944 

415,842,698 

1909 

81,070,359 

379,744,257 

460,814,616 

1910 

84,485,236 

417,111,142 

501,596,378 

1911 

90,464,067 

405,907,059 

496,371,126 

1912 

84,361,598 

450,104,982 

534,466,580 

1913 

91,524,922 

478,435,297 

569,960,219 

1914 

90,821,507 

422,703,970 

513,525,477 

1915 

88,995,061 

442,624,426 

531,619,487 

2,626,512,578 

8,262,792,323 

10,889,304,901 

COAL  PRODUCING  STATES 

Coal  is  produced  in  thirty  states,  but  almost  eighty  (80%) 
per  cent,  is  produced  west  of  the  Mississippi  river.  The  relative 
importance  of  the  states  was  expressed  by  the  Geological  Sur- 
vey in  percentage  of  all  coal  produced  (bituminous  and  anthra- 
cite) as  follows,  for  1915: 


88  MECHANICAL  STOKERS 

Pennsylvania : 

Anthracite    16.8% 

Bituminous  29.7% 

West  Virginia 14.5% 

Illinois 11.1% 

Ohio 4.2% 

Kentucky    4.0% 

Indiana  3.2% 

Alabama  2.8% 

Colorado    1.6% 

Virginia    1.5% 

Maryland    8% 

Oklahoma    8% 

Missouri    8% 

New  Mexico 7% 

Utah    6% 

Washington    6% 

Montana   5% 

Texas   4% 

Arkansas 3% 

Michigan 2% 

Iowa    1.4% 

Kansas    1.3% 

Wyoming    1.2% 

North  Dakota    1% 

Georgia,  Oregon,  California,  Idaho,  Nevada,  South  Dakota. .       .1% 

COMPOSITION  OF  COALS 

To  some  extent,  the  analysis  of  a  coal  will  give  some  indi- 
cation of  how  successfully  it  can  be  handled  on  a  grate  or 
stoker.  These  analyses  can  be  made  with  proper  facilities,  and 
are  not  too  technical  for  boiler-room  work.  The  two  analyses 
generally  used  are  the  Proximate  analysis  and  Ultimate 
analysis. 

Proximate  Analysis. — The  proximate  analysis  of  coal  de- 
termines the  moisture,  volatile  matter,  fixed  carbon,  percentage 
of  ash,  and  sulphur  (separately  determined).  The  fixed  carbon 
is  the  carbon  remaining  after  distillation.  Volatile  matter  is 
the  total  combustible,  less  the  carbon  contents,  and  includes 
hydric  carbons,  etc.  Ash  is  the  residue  remaining  after  the 
moisture  and  volatile  contents  have  been  driven  off  and  the 
carbon  consumed.  Moisture  is  that  percentage  of  the  weight 
of  the  coal  when  dried  at  a  given  temperature. 


COAL  AND  COAL-PRODUCING  FIELDS  89 

Ultimate  Analysis. — The  ultimate  analysis  is  a  more  compli- 
cated chemical  analysis  giving  the  percentages  of  carbon  (C), 
hydrogen  (H),  nitrogen  (N),  Sulphur  (S),  and  ash  (A).  This 
analysis  is  used,  and  is  necessary  for  making  the  heat  balance 
on  any  given  boiler  test.  The  ultimate  analysis  does  not  dis- 
tinguish between  the  carbon  and  hydrogen  derived  from  the 
combustible  matter  of  coal. 

Heating  Value. — The  heating  value,  sometimes  called  the 
calorific  value  of  coal,  is  the  number  of  units  of  heat  liberated 
by  the  perfect  combustion  of  a  unit  weight  of  the  coal.  The 
British  Thermal  Unit  (B.T.U.)  is  used  to  designate  the  heating 
value,  and  is  the  quantity  of  heat  required  to  raise  the  tem- 
perature of  one  pound  of  water  1°  F.  A  bomb  calorimeter, 
many  of  which  are  on  the  market,  is  used  to  determine  the 
heating  value  of  coal,  and  can  easily  be  used  in  connection  with 
boiler-room  equipment. 

Heating  Value  from  Analysis. — A  number  of  equations  have 
been  derived  to  give  the  heating  value  of  coal  from  the  analy- 
sis. The  one  most  commonly  used  is  DuLong's  formula,  which 
is  as  follows : 

B.T.U.  per  Ib.  of  coal  =  14,544  C + 62,028 (H-^  +4050  S. 

C,  H,  0  and  S  are  the  percentages  of  carbon,  hydrogen, 
oxygen  and  sulphur,  respectively,  in  the  combustible. 

For  western  coals,  DuLong's  formula  gives  heating  values 
a  little  too  low,  but  for  eastern  coals,  is  sufficiently  close  for 
estimating  purposes. 

CLASSIFICATION  OF  COALS 

U.  S.  Geological  Survey.— Coals  can  be  classified  in  many 
different  ways,  namely,  according  to  the  chemical  composition, 
the  ratio  of  volatile  and  carbon,  location  of  mines,  etc.  The 
United  States  Geological  Survey  method  of  classifying  coals 
is  part  chemical  and  part  physical,  the  same  being  as  follows: 

Anthracite  is  generally  defined  as  hard  coal,  most  of  it  being 
mined  in  Eastern  Pennsylvania.  Small  areas  of  anthracite 
occur  in  the  West.  Anthracite  is  an  almost  ideal  domestic  fuel, 
but  largely  on  account  of  its  low  heating  power,  it  is  not  an 


90  MECHANICAL  STOKERS 

economical  fuel  for  steam  raising  or  for  use  in  general  manu- 
facturing. 

Semi- Anthracite  is  also  a  hard  coal,  but  it  is  not  so  hard 
as  true  anthracite.  It  is  high  in  fixed  carbon,  but  not  so  high 
as  anthracite.  The  change  of -ordinary  soft  coal  to  semi-anthra- 
cite is  due  to  the  same  causes  that  produced  anthracite,  except 
that  the  process  has  not  been  carried  so  far  in  semi-anthracite. 
There  is  very  little  semi-anthracite  in  this  country,  so  it  is  only 
a  small  factor  in  the  coal  trade.  Such  semi-anthracite  as  is 
mined  reaches  the  consumer  generally  under  the  name  ' '  anthra- 
cite." 

Semi-Bituminous. — The  name  "semi-bituminous"  is  exceed- 
ingly unfortunate,  as  literally  it  implies  that  this  coal  is  half 
the  rank  of  bituminous,  whereas,  it  is  applied  to  a  kind  of 
coal  that  is  of  higher  rank  than  bituminous — really  super- 
bituminous.  Its  relatively  high  percentage  of  fixed  carbon 
makes  it  nearly  smokeless  when  it  is  burned  properly,  and  con- 
sequently, most  of  these  coals  go  into  the  market  as  ' '  smokeless 
<oals. ' '  The  best  coal  of  this  type  has  a  heating  value  greater 
than  that  of  any  of  the  other  ranks,  and  is,  consequently,  best 
adapted  to  raising  steam  and  to  general  manufacturing  that 
requires  a  high  degree  of  heat.  It  is  regarded  as  the  best  coal 
for  steamship  and  especially  for  naval  use,  as  it  is  nearly  smoke- 
less and  requires  less  bunker  space  per  unit  of  heat  than  other 
coals.  The  coal  is  generally  minutely  jointed  and  is,  therefore, 
tender  and  friable.  In  fact,  it  is  so  friable  that,  in  mining,  a 
large  percentage  of  fine  coal  is  produced,  and  in  transporta- 
tion, many  of  the  lumps  are  broken  to  pieces,  so  that  by  the 
time  it  reaches  the  consumer,  especially  if  it  has  been  trans- 
shipped, it  is  generally  in  small  pieces.  This  fineness,  is,  by 
many,  regarded  as  detrimental  because  the  public  is  accustomed 
to  lump  coal  which  will  stand  transportation  without  crushing, 
but  when  this  coal  is  used  with  mechanical  stokers  and  with 
a  grate  adapted  to  its  use,  the  fineness  of  the  coal  is  not  dis- 
advantageous. The  great  bulk  of  this  kind  of  coal  is  in  the 
eastern  fields,  but  some  is  found  in  the  West. 

Bituminous. — The  term  "bituminous,"  as  generally  under- 
stood, is  applied  to  a  group  of  coals  in  which  the  volatile  matter 
and  the  fixed  carbon  are  nearly  equal ;  but  this  criterion  cannot 


COAL  AND  COAL-PRODUCING  FIELDS  91 

be  used  without  qualification,  for  the  same  statement  might  be 
made  of  sub-bituminous  coal  and  lignite.  As  noted  before,  the 
distinguishing  feature  which  serves  to  separate  bituminous 
coal  from  coals  of  lower  rank  is  the  manner  in  which  it  is 
affected  by  weathering.  Bituminous  coal  is  only  slightly  af- 
fected chemically  by  weathering  unless  it  is  exposed  for  many 
years,  and  then,  although  it  consists  of  small  particles,  each 
particle  is  a  prismatic  fragment,  whereas  coals  of  lower  rank 
break  into  thin  plates  parallel  with  the  bedding. 

Suit-Bituminous. — The  term  "Sub-bituminous"  is  adopted 
by  the  Geological  Survey  for  what  has  generally  been  called 
" black  lignite,"  a  term  that  is  objectionable  because  the  coal 
is  not  lignite  in  the  sense  of  being  distinctly  woody.  Sub-bitu- 
minoiis  coal  is  generally  distinguishable  from  lignite  by  its  black 
color  and  its  apparent  freedom  from  distinctly  woody  texture 
and  structure,  and  from  bituminous  coal  by  its  loss  of  moisture 
and  the  consequent  breaking  down  or  "slacking"  that  it  un- 
dergoes when  subjected  to  alternate  wetting  and  drying.  As 
the  percentage  of  moisture  is  an  important  matter  in  buying 
and  shipping  coal,  and  as  the  slacking  on  exposure  to  the 
weather  makes  it  necessary  to  ship  in  box  cars  and  to  guard 
carefully  against  spontaneous  ignition,  there  is  a  great  com- 
mercial difference  in  these  two  kinds  of  coal  which  the  Geo- 
logical Survey  has  recognized  by  putting  them  in  different 
ranks.  Despite  the  many  drawbacks  in  the  shipment  and  use 
of  sub-bituminous  coal,  it  has  found  a  ready  market  in  much 
of  the  western  country,  because  it  is  a  very  clean  domestic 
fuel  and  ignites  with  little  difficulty. 

Sub-bituminous  coals  differ  considerably  in  chemical  com- 
position and  in  physical  appearance.  Some  are  banded  like 
much  of  the  bituminous  coal,  and  some  are  essentially  cannel 
in  physical  and  chemical  make-up.  In  general,  the  younger 
coals  of  the  West  contain  a  smaller  percentage  of  sulphur  than 
the  older  coals  of  the.  East,  and  some  of  them  are  high  in  vola- 
tile matter. 

Lignite. — The  term  "lignite,"  as  used  by  the  Geological 
Survey,  is  restricted  to  those  coals  which  are  distinctly  brown 
and  either  markedly  woody  or  claylike  in  their  appearance. 
They  are  intermediate  in  quality  and  in  development  between 


92 


MECHANICAL  STOKERS 


peat  and  sub-bituminous  coal.  As  the  moisture  of  lignite  as 
it  comes  from  the  mine  generally  ranges  from  30  to  40  per  cent, 
its  heating  value  is  low,  and  the  consumer  cannot  afford  to 
pay  freight  for  any  great  distance  on  so  much  water.  Also, 
it  parts  with  much  of  this  moisture  very  readily  when  exposed 
to  the  weather  and  so  falls  to  pieces  or  slacks  much  more  readily 
and  completely  than  sub-bituminous  coal.  On  this  account,  it 
is  more  likely  to  ignite  spontaneously  and  must  be  handled 
even  more  carefully  than  sub-bituminous  coal  and  stored  in  a 
place  where  it  will  not  be  exposed  to  alternate  wetting  and 
drying.  Lignite  is  mainly  marketed  near  the  mine,  as  a  domes- 
tic fuel,  but  at  a  few  places  in  North  Dakota  and  Texas,  it  is 
shipped  to  nearby  towns  and  used  for  general  manufacturing 
purposes. 

Lignite  can  be  manufactured  into  hard  briquets,  which 
make  an  excellent  fuel,  but  so  far,  the  cost  of  manufacture  has 
been  prohibitive. 

Wm.  Kent  Classification. — A  classification  of  American 
coals,  based  on  the  proximate  and  ultimate  analyses  and  heat- 
ing values  of  155  coals  from  different  States  selected  from  the 
analyses  of  over  3,000  coals  published  in  Bulletin  22  of  the 
U.  S.  Bureau  of  Mines,  is  as  follows: 


CLASSIFICATION  AND  HEATING  VALUE  OF  COALS 


Volatile 

Matter, 

Oxygen 

Moisture  in 

B.T.U. 

B.T.U. 

Per  Cent 

in  Com- 

Air Dry  Coal 

per  Pound 

of  Com- 
bustible 

bustible 
Per  Cent 

Free  from 
Ash,  Per  Cent 

Combustible 

Coal  Air 
Dry  Ash  Free 

I.  Anthracite  

Less  than 

Ito4 

Less  than  1  .  8 

14,800  to  15,400 

14,600  to  15,400 

10 

10 

II.  Semi-anthracite. 

10  to  15 

Ito5 

Less  than  1  .  8 

15,400  to  15,500 

15,200  to  15,500 

III.  Semi-bituminous 

15  to  30 

Ito6 

Less  than  1  .  8 

15,400  to  16,050 

15,300  to  16,000 

IV    Cannel*        

45  to  60 

5to8 

Less  than  1  .  8 

15,700  to  16,200 

15,500  to  16,050 

V.  Bituminous, 

high  grade  

30  to  45 

5  to  14 

Ito4 

14,800  to  15,600 

14,350  to  15,500 

VI.  Bituminous,  me- 

dium grade.  .  .  . 

32  to  50 

6  to  14 

2.5to6.5 

13,800  to  15,100 

13,400  to  14,400 

VII.  Bituminous,  low 

grade  

32  to  50 

7  to  14 

5  to  12 

12,400  to  14,600 

11,300  to  13,400 

VIII.  Sub-bituminous 

and  lignite  

27  to  60 

10  to  33 

7  to  26 

9,600  to  13,250 

7,400  to  11,650 

*  Eastern  cannel.     The  Utah  cannel  is  much  lower  in  heating  value. 


COAL  AND  COAL-PRODUCING  FIELDS 


93 


Classes  I,  II  and  III  are  the  same  as  in  earlier  classifications, 
the  semi-bituminous  coals  containing  between  15  and  30  per 
cent  of  volatile  matter  in  the  combustible.  Classes  V,  VI  and 
VII  have  heretofore  been  considered  as  a  single  class,  but  they 
vary  greatly  in  heating  value  and  in  the  amount  of  moisture 
remaining  in  air-dried  coal,  which  is  used  as  the  basis  of  the 
sub-division  into  three  classes.  Class  VIII  includes  the  two 
classes  sub-bituminous  and  lignite  of  the  U.  S.  Geological  Survey, 
which  are  differentiated  one  from  the  other  by  color,  texture 
and  disintegration  by  weathering,  but  not  by  heating  value  or 
by  analysis. 

COMPOSITION  OF  TYPICAL  COALS 


Trade  Names  of  Coals  and  Location 

PROXIMATE  ANALYSIS 

B.T.U. 

value 
as 
Fired 

Moist- 
ure 

Volatile 
Matter 

Fixed 
Carbon 

Ash 

Sulphur 

EASTERN  COALS 
New  River,  W.  Va  

2.14 
2.09 
2.00 
1.45 
.56 

1.61 
1.37 
1.76 
7.24 

6.02 
.69 
1.12 

9.04 
7.12 
7.92 
7.90 
11.90 
1.40 
12.10 

18.80 
40.50 
28.90 
22.60 
12.10 
18.00 
14.48 

21.73 
14.70 
26.20 
19.25 
16.71 

32.19 
31.70 
33.36 
30.47 

30.29 
31.76 
29.64 

32.63 
39.78 
35.98 
30.97 
31.50 
35.12 
36.30 

30.50 
26.30 
35.90 
32.50 
36.80 
31.80 
33.51 

71.56 
76.75 
66.50 
70.09 
76.22 

55.94 
57.00 
53.62 
51.41 

52.34 

57.78 
58.74 

47.01 
43.70 
46.93 
47.75 
49.80 
49.50 
42.90 

40.50 
27.00 
27.30 
40.40 
40.70 
39.70 
34.60 

4.57 
6.46 
5.30 
9.21 
6.51 

10.26 
9.93 
11.26 

10.88 

11.35 
9.77 
10.50 

11.32 
9.40 
9.17 
13.38 
6.80 
13.98 
8.70 

10.20 
6.20 
7.90 
4.50 
10.40 
10.50 
17.41 

.80 
.71 
1.20 
1.42 
1.10 

1.33 
.80 
2.91 
1.26 

1.85 
2.02 
1.10 

2.80 
1.81 
4.90 
3.62 
1.20 
3.10 
3.9 

.60 
.80 
.50 
1.40 
.30 
2.20 
4.50 

14,690 
14,520 
13,995 
14,451 
14,279 

13,561 
13,317 
12,800 
11,777 

12,460 
13,410 
13,564 

11,650 
11,830 
12,258 
11,507 
11,780 
13,020 
11,450 

9,650 
10,121 
8,000 
12,316 
12,590 
8,910 
9,572 

Pocahontas,  W.  Va  

Cabin  Creek   W   Va 

Georges  Creek   Md 

Kittunning  run-of-mine   Pa 

PITTSBURGH  COALS 
Pittsburgh  run-of-mine,  Pa  
Youghiogheny  run-of-mine,  Pa       .... 

Fairmont  run-of-mine  W   Va          .... 

Hocking  Valley  lump,  Ohio  
Buffalo,     Rochester    and     Pittsburgh 
slack,  Pa                               

Shawmut  slack,  Pa      

Reynoldsville  slack,  Pa  

MIDDLE  WESTERN  COALS 
Carterville  mine-run,  111  

Seeleyville  mine-run,  Ind  

Eastern  mine-run   Tenn 

Missouri  City  coal   Mo  .  .  . 

SuB-BlTUMINOTTS  AND  LlGNITES 

Denver  mine-run,  Colo  

Lehigh  mine-run,  N.  D  

New  Castle  mine  Wash 

New  Market  mine   Iowa    

94  MECHANICAL  STOKERS 

Commercial  Classification. — The  classification  of  coals,  as 
adopted  by  the  United  States  Geological  Survey,  seems  to  suit 
the  particular  conditions  under  which  the  Survey  operates,  but 
combustion  engineers  and  mechanical  stoker  manufacturers 
classify  coals  more  according  to  their  action  to  coke  or  fuse 
when  burned  on  a  grate.  The  following  classification  is 
generally  used  in  connection  with  stoker  work : 

Eastern  Bituminous  Coal. — This  embraces  the  bituminous 
coals  of  Virginia,  Eastern  Pennsylvania,  West  Virginia  and 
Maryland.  The  coal  is  a  caking  coal,  low  volatile,  low  ash,  high 
fixed  carbon  with  about  the  following  proximate  analysis : 

Moisture    1%  to  5% 

Volatile  Matter 16%  to         22% 

Fixed  Carbon 68%  to         77% 

Ash    3%  to         10% 

Sulphur    g%  to        1.5% 

B.T.U 14,000      to  15,000 

Pittsburgh  Coal. — This  coal  embraces  the  bituminous  coals 
of  Western  Pennsylvania,  Eastern  Ohio,  Eastern  Kentucky  and 
part  of  West  Virginia.  This  is  a  coking  coal,  high  in  volatile 
matter,  with  about  the  following  proximate  analysis : 

Moisture   3%  to          8% 

Volatile   Matter    32%  to        38% 

Fixed  Carbon    56%  to         60% 

Ash    9%  to         12% 

Sulphur 1.5%  to           3% 

B.T.U 13,500       to  14,200 

Middle  West  Coal. — Most  of  the  Middle  West  coals,  coming 
from  Illinois,  Michigan  and  Indiana,  are  free  burning  and  of 
high  ash  content.  These  coals  have  about  the  following  proxi- 
mate analysis : 

Moisture  8%  to        14% 

Volatile  Matter 28%  to        35% 

Fixed  Carbon    38%  to         53% 

Ash    8%  to         20% 

Sulphur 3%  to  4% 

B.T.U 11,300       to  12,500 


COAL  AND  COAL-PRODUCING  FIELDS  95 

Eastern  Kentucky,  Tennessee  and  Alabama  Coal. — Most  of 
the  coal  from  this  group  is  free  burning  and  relatively  high  in 
heating  value.  They  have  about  the  following  proximate 
analysis : 

Moisture   5%  to         10% 

Volatile  Matter  30%  to         37% 

Fixed   Carbon    50%  to         60% 

Ash    5%  to         15% 

B.T.U 13,000  to  13,500 

Texas,  Oklahoma  and  Arkansas  Coal. — Many  different  kinds 
of  coals  are  found  in  these  states,  ranging  from  semi-anthracite 
to  lignites,  these  coals  having  about  the  following  proximate 
analysis : 

(a)   Semi-anthracite — 

Volatile  Matter 10%  to  16% 

Fixed   Carbon    70%  to  77% 

Ash    8%  to  18% 

B.T.U 13,500       to  14,000 

(&)   Bituminous — 

Volatile  Matter  30%  to  35% 

Fixed   Carbon    30%  to  40% 

Ash    10%  to  20% 

B.T.U 11,500       to  12,500 

(c')   Lignite — 

Moisture 20%  to  35% 

Volatile  Matter 30%  to  40% 

Fixed  Carbon    30%  to  35% 

Ash    10%  to  15% 

B.T.U.                                                                7000       to  9000 


Colorado  Coals. — The  coals  produced  and  used  in  Colorado 
range  from  anthracite  to  bituminous  and  lignite.  In  the  western 
part  of  the  state,  the  coal  is  high  volatile  and  free  burning.  The 
coal  in  the  southern  part  of  the  state  represents  very  much  the 
Pittsburgh  coals  inasmuch  as  it  is  a  highly  coking  coal.  In  the 
northwestern  part  of  the  state,  some  anthracite  coal  is  used. 

What  is  generally  termed  "  Denver  Lignite  "  ( coals  of  lignitic 
nature  coming  from  the  vicinity  of  Denver)  is  very  high  in 


96  MECHANICAL  STOKERS 

moisture  and  slacks  when  heated,  and  has  about  the  following 
proximate  analysis: 

Denver  Lignite — 

Moisture   18%  to  20% 

Volatile  Matter   30%  to  36% 

Fixed  Carbon    36%  to  44% 

B.T.U 9000       to  9600 

Sulphur     2%   to  3% 

Ash    6%  to  8% 

Washington,  Wyoming  and  Montana  Coal. — Some  of  the  coals 
from  Washington,  especially  Eoslyn,  are  bituminous  coals  of  high 
grade,  the  same  having  the  following  proximate  analysis : 

Roslyn,  Wash.:  Rock  Springs,  Wyo.:  Nelson,  Mont.: 

Moisture ...     3 . 77%        Moisture 9.8%  Moisture ...  3 . 60% 

Vol.  mat. ...  37 . 69%  Vol.  mat ....  34 . 3%  Vol.  mat. ...  28 . 52% 

F.  C 47.05%        F.  C 52.5%        F.  C 46.41% 

Ash 11.49%        Ash 3.4%        Ash 21.47% 

Sulphur 47%        Sulphur 1.0%        Sulphur 

B.T.U 12,767        B.T.U 12200          B.T.U 10,447 

North  Dakota  Lignite. — This  coal  is  of  lignitic  nature,  con- 
taining a  high  percentage  of  moisture  and  a  low  heating  value, 
and  very  little,  if  any,  sulphur.  An  average  proximate  analysis 
of  this  coal  from  one  of  the  best  mines  is  as  follows : 

Moisture 30. 6% 

Volatile  matter 31 .9% 

Fixed  carbon 32 . 2% 

Ash 5.3% 

B.T.U 8058 

Coke  Breeze. — Coke  breeze  is  the  refuse  that  is  taken  from 
coke  ovens  whenever  they  are  drawn.  For  some  time,  this  fuel 
had  no  value  but  is  now  being  used  successfully  for  steam  pur- 
poses. There  are  two  kinds  of  coke  breeze,  one  coming  from 
beehive,  and  the  other  from  by-product  coke  ovens.  Some  is 
what  is  called  48-hour  drawn,  and  others  72-hour  drawn,  and 
there  is  quite  a  difference  between  the  two  fuels.  This  fuel  has 
the  following  proximate  analysis : 


COAL  AND  COAL-PRODUCING  FIELDS  97 

48-hour  drawn       72-hour  drawn 

Moisture 3.51  5.24 

Volatile  matter 12. 58  6. 07 

Fixed  carbon 58.66  62. 13 

Ash 28 . 76  31 . 80 

Sulphur 083  1.02 

B.T.U 11,017  10,323 

The  average  coke  breeze  contains  from  25%  to  35%  sand, 
sulphur  and  fire-clay  which  tend  to  make  clinkers  when  the  fuel 
is  heated. 

Anthracite  Culm. — Most  of  the  anthracite  coal  produced  is  used 
for  domestic  purposes,  but  the  fine  anthracite  screenings  have, 
for  some  years,  been  a  waste  product  and  thrown  out  in  great 
piles  in  the  anthracite  mining  regions.  These  screenings  have 
been  used  successfully  on  some  types  of  stokers,  and  have  also 
been  used  when  mixed  with  bituminous  coal.  The  sizes  most 
generally  and  successfully  used  are  Nos.  2  and  3  Buckwheat,  No. 
2  Buckwheat  having  about  the  following  proximate  analysis : 

Ash.... 21% 

Moisture 8% 

B.T.U 10,500 

The  screening  sizes  used  generally  for  anthracite  coal  are  as 
follows : 

No.  1  buckwheat  through  &"  over  YS" 
No.  2          "  "       &"    "    &" 

No.  3          "  "       &"    "    &" 

No.  4  "       A"    "    A" 

Bone  Coal. — The  refuse  from  coal  mining  consisting  of  slate 
and  bone  coal  was,  for  many  years,  thrown  out  in  great  piles 
and  considered  a  waste  product.  This  refuse,  however,  had  been 
used  in  connection  with  stoker-fired  furnaces.  It  is  very  high  in 
ash  and  low  in  heat  value  with  about  the  following  proximate 
analysis : 

Moisture 3%  to    6% 

Volatile  matter 15%  to  20% 

Fixed  carbon 20%  to  25% 

Ash 25%  to  50% 

Sulphur 1%  to    2% 

B.T.U 3000  to  6000 


98 


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CHAPTER   IV 

COMBUSTION     CHARACTERISTICS     OF     COAL    AND 
SELECTION  OF  SUITABLE  STOKER  EQUIPMENT 

The  United  States  produces  every  variety  of  coal  from  an- 
thracite to  lignite.  These  different  coals  possess  widely  differ- 
ing characteristics  requiring  conditions  peculiar  to  each  par- 
ticular coal  for  the  best  combustion  results,  and  to  meet  these 
conditions,  a  great  variety  of  mechanical  stokers  have  been 
developed.  Each  stoker  has  been  developed  to  meet  some  cer- 
tain set  of  conditions  and  after  proving  successful,  has  gradu- 
ally enlarged  its  field  of  application. 

In  general,  it  may  be  said  that  any  stoker  will  burn  practi- 
cally every  coal  with  some  degree  of  success,  but  that  no  stoker 
is  a  commercial  success  with  every  coal.  It  is  necessary,  there- 
fore, to  determine  for  a  given  coal,  those  stokers  which  will 
burn  it  successfully  and  exclude  from  consideration  those  not 
suited.  Stokers  have  proven  unsatisfactory  in  a  greater  per- 
centage of  cases  than  any  other  piece  of  power  plant  apparatus, 
and  the  great  majority  of  these  failures  were  due  to  improper 
selection  rather  than  to  any  inherent  weakness  of  the  stoker 
selected. 

it  is  necessary  as  a  preliminary  to  the  selection  of  a  stoker 
to  divide  those  available  into  groups,  as  given  in  Chapter  2, 
and  to  determine  the  adaptability  of  these  groups  to  burning 
the  different  grades  of  coal. 

It  is  also  necessary  to  explain  what  will  be  understood  here- 
after by  certain  terms  which  are  commonly  used  in  a  rather 
indefinite  manner. 

Combustion  Rates. — Many  curves  have  been  plotted  show- 
ing the  relation  between  fuel  bed  resistance  and  combustion 
rates  for  the  various  grades  of  fuel  but  it  will  often  be  found 
that  the  results  of  an  actual  test  differ  greatly  from  the  com- 
monly given  values.  The  reason  for  this  is  the  varying  per- 

106 


COMBUSTION  CHARACTERISTICS  OF  COAL  107 

centage  of  dust  and  small  particles  in  the  coal  as  well  as  the 
amount  of  surface  moisture  present.  The  amount  of  dust  which 
any  coal  contains  will  depend  upon  the  structure  of  the  coal 
and  the  amount  of  handling  it  has  received.  Coal  which  is 
very  brittle  will  often  be  received  containing  a  very  large  per- 
centage of  dust  whereas  the  harder  grades  will  contain  a  com- 
paratively small  percentage  of  dust.  The  amount  of  dust 
present  has  a  decided  bearing  on  the  resistance  to  the  flow  of 
air,  and  this  is  one  of  the  principal  reasons  why  the  fuel  bed 
resistance  in  a  particular  case  may  differ  largely  from  the 
average  values  usually  given.  A  fuel  containing  30%  of  par- 
ticles which  will  pass  through  a  one-eighth  inch  round  hole, 
will  offer  about  50%  greater  resistance  to  the  flow  of  air  than 
a  fuel  containing  no  such  particles.  This  condition  does  not  exist 
after  the  entire  mass  of  fuel  is  in  active  combustion  but  it  does 
retard  ignition  and  appreciably  reduces  the  capacity  that  can  be 
secured  under  a  given  set  of  conditions.  This  excessive  fuel 
bed  resistance  can  be  largely  overcome  .by  adding  sufficient  sur- 
face moisture  to  agglomerate  the  dust.  This  water  should  not 
be  added  in  the  stoker  hopper  but  should  be  mixed  with  the 
coal  before  it  is  delivered  to  the  coal  bunkers.  The  water  then 
has  an  opportunity  to  become  evenly  distributed  through  the 
entire  mass  and  produce  the  desired  result  without  requiring 
an  excessive  amount  of  moisture. 

When  the  correct  amount  of  moisture  has  been  added  and 
thoroughly  mixed  with  the  coal,  it  should  stick  together  when 
tightly  squeezed  in  the  hand.  The  addition  of  moisture  makes 
the  fuel  bed  more  porous,  not  more  compact,  as  is  commonly 
supposed.  The  rapid  evaporation  of  this  moisture  when  the 
fuel  is  introduced  into  the  furnace  produces  steam  which  tends 
to  keep  the  mass  broken  up  and  porous.  It  is  for  this  reason, 
the  wetting  of  certain  coals  greatly  assists  in  their  combustion. 
From  5%  to  8%  surface  moisture  is  usually  sufficient  and  the 
improved  fuel  bed  conditions  which  result  more  than  offset  the 
loss  produced  by  the  evaporation  of  this  percentage  of  moisture. 

It  is  unfortunate  that  the  practice  of  making  a  screening 
test  and  a  surface  moisture  determination  is  not  a  part  of  every 
boiler  test.  Such  data  would  assist  in  explaining  a  number  of 
apparent  discrepancies  in  the  results  secured. 


108  MECHANICAL  STOKERS 

Eastern  Coals. — The  low  volatile,  low  ash,  high  carbon  coals 
of  Pennsylvania,  West  Virginia,  and  Virginia,  form  a  well  de- 
fined class.  They  contain  from  16%  to  22%  volatile,  68%  to  77% 
fixed  carbon,  3%  to  10%  ash,  2%  to  6%  moisture,  and  from 
14,200  to  15,000  B.T.U.  per  Ib.  dry.  The  peculiarity  of  such  coal 
is  that  when  subjected  to  high  temperature,  it  swells  and  tends 
to  cake  together  into  a  solid  mass.  For  satisfactory  combustion 
it  is  necessary  that  the  fuel  bed  be  uniform  and  porous  enough 
to  allow  the  air  required  for  combustion  to  pass  through  and 
it  is  therefore  necessary  to  agitate  this  coal,  especially  during 
the  early  stages  of  combustion,  to  secure  the  best  results.  For- 
tunately, the  fusing  temperature  of  the  ash  is  very  high 
(2500°  F.  or  higher)  and  the  ash  content  low,  so  that  the 
necessary  agitation  does  not  cause  clinker  trouble. 

Side  feed  and  front  feed  stokers  provide  a  sufficient  agita- 
tion to  keep  the  fuel  bed  broken  up  in  a  uniform  and  porous 
condition  and  therefore  can  burn  this  coal  satisfactorily  if  the 
proper  adjustments  are  maintained  and  the  fire  not  allowed  to 
become  too  heavy.  In  the  case  of  a  heavy  fuel  bed,  the  action 
of  the  grates  is  not  sufficient  to  agitate  the  mass  throughout  its 
full  depth,  and  the  surface  becomes  caked  to  such  an  extent 
that  the  free  passage  of  air  is  not  possible. 

On  account  of  the  high  percentage  of  fixed  carbon  this  coal 
is  slow  burning  and  the  amount  that  can  be  burned  with  a 
given  draft  is  less  than  in  the  case  of  other  bituminous  coals. 
Natural  draft  is  usually  employed  with  this  type  of  stoker, 
and  with  .50"  draft  over  the  fire,  which  is  about  as  much  as  is 
usually  available,  combustion  rates  of  more  than  35  Ibs.  per  sq. 
ft.  of  grate  surface  per  hour  cannot  be  secured  without  exces- 
sive loss  of  combustible  to  the  ash  pit.  Forced  draft  can  be 
applied  but  a  heavier  fire  is  then  required  for  best  results  and 
if  the  thickness  is  greater  than  about  6"  the  grate  motion  is 
not  sufficient  to  keep  the  surface  of  the  fuel  bed  properly 
broken  up.  Sufficient  grate  surface  should  therefore  be  in- 
stalled to  burn  the  maximum  amount  of  coal  required  at  a  rate 
not  greater  than  35  Ibs.,  and  under  these  conditions,  very  satis- 
factory performances  should  be  secured.  There  should  be  no 
serious  clinker  trouble  and  on  those  stokers  equipped  with 


COMBUSTION  CHARACTERISTICS  OF  COAL  109 

continuous  ash  removing  mechanism,  the  cleaning  is  automatic 
and  continuous. 

All  stokers  of  the  underfeed  type  are  well  adapted  for  burn- 
ing Eastern  coal.  The  tendency  of  the  coal  to  swell  accentuates 
the  action  of  the  stoker  to  produce  a  fuel  bed  of  unusual  thick- 
ness (24"  or  greater)  and  the  pushing  action  of  the  feeding 
mechanism  keeps  the  fuel  bed  broken  up  and  porous.  The  fuel 
bed  is  in  the  proper  condition  for  the  employment  of  high  air 
pressure  and  combustion  rates  of  60  Ibs.  per  sq.  ft.  of  grate  area 
can  be  maintained  for  long  periods  of  time  and  about  70  Ibs.  per 
sq.  ft.  for  two  or  three  hour  peaks.  The  high  fusing  tempera- 
ture of  the  ash  and  the  low  percentage  of  ash  in  the  coal  com- 
bine to  make  the  cleaning  periods  infrequent  and  of  short  dura- 
tion. High  boiler  ratings  can  be  carried  on  account  of  the  high 
combustion  rates  possible  and  the  high  heating  value  of  the 
coal,  and  unusually  good  efficiency  of  combustion  is  secured 
on  account  of  the  adaptability  of  the  coal  to  the  fuel  bed  con- 
ditions, secured  with  this  type  of  stoker. 

The  fuel  bed  conditions  are  ideal  for  banking  and  quickly 
bringing  banked  boilers  into  service  at  high  ratings.  A 
properly  banked  unit  can  be  brought  up  to  200%  rating  in  four 
or  five  minutes,  thus  providing  a  degree  of  flexibility  which  is 
highly  essential  in  certain  types  of  central  stations.  The  fuel 
bed  can  be  comparatively  close  to  the  boiler  heating  surface 
without  producing  smoke  and  a  large  amount  of  heat  is  trans- 
mitted from  the  fuel  bed  to  the  boiler  by  direct  radiation. 

Traveling  grates  unless  provided  with  means  for  agitating 
the  fuel  during  the  early  stages  of  combustion  will  not  give 
satisfactory  results  with  Eastern  coal.  The  fuel,  being  carried 
from  front  to  rear  of  the  furnace  without  agitation,  tends  to 
form  a  solid  mass  of  coke  which  seriously  interferes  with  the 
supply  of  air  through  the  fuel  bed.  The  air  will  finally  break 
through  in  spots  and  the  fuel  will  burn  intensely  but  unevenly 
at  the  rear  of  the  furnace.  The  result  is  that  large  amounts 
of  unburned  coke  are  discharged  to  the  ash  pit  and  the  grates 
are  overheated  in  spots  where  the  air  does  not  break  through. 
Agitating  devices  have  been  applied  to  the  traveling  grate 
which  break  up  the  mass  of  coke  and  produce  a  fairly  uniform 
and  porous  fuel  bed  and  satisfactory  operation  can  be  secured. 


110  MECHANICAL  STOKERS 

Combustion  rates  up  to  40  Ibs.  per  sq.  ft.  per  hour  can  be  secured 
with  this  arrangement,  but  above  this  rate,  the  ash  pit  loss 
increases  rapidly  and  burning  of  the  grate  surface  becomes 
serious.  Further  development  along  this  line  must  depend 
upon  the  application  of  forced  draft.  This  requires  a  different 
design,  as  attempts  to  apply  forced  draft  to  the  standard 
stokers  of  this  type  have  generally  proven  unsuccessful. 

Experience  with  traveling  grates  has  demonstrated  that 
while  agitation  of  Eastern  coal  is  desirable  it  is  not  absolutely 
necessary.  It  is  possible  by  carrying  a  thin  fire  and  a  strong 
draft,  to  develop  a  region  of  high  temperature  where  the  fuel 
enters  the  furnace,  and  consume  the  volatile  without  caking 
the  solid  particles  in  the  fuel,  but  the  thickness  of  fuel  bed 
is  not  sufficient  for  good  operating  results.  By  the  application 
of  forced  draft,  the  fuel  bed  can  be  increased  to  the  proper 
thickness  and  the  air  supply  regulated  to  secure  the  desired 
temperature  and  combustion  at  the  front  of  the  furnace.  Under 
these  conditions  satisfactory  results  are  to  be  expected. 

Pittsburgh  Coal. — The  high  volatile  coals  of  Western  Penn- 
sylvania, Eastern  Ohio,  Eastern  Kentucky,  and  parts  of  West 
Virginia  may  be  grouped  under  the  heading  of  Pittsburgh  coal. 
They  contain  from  30%  to  38%  volatile,  50%  to  60%  fixed 
carbon,  5%  to  12%  ash,  3%  to  10%  moisture  and  from  13,200 
to  14,000  B.T.U.  per  Ib.  dry.  All  these  coals  possess  caking 
qualities  to  a  greater  or  less  degree  but  not  to  such  an  extent 
as  the  Eastern  coals,  although  there  is  no  clear  line  of  division 
between  them. 

Overfeed  stokers  burn  this  coal  very  satisfactorily  if  pro- 
vided with  proper  draft  and  setting  conditions.  The  caking 
tendencies  are  not  sufficient  to  cause  trouble  if  proper  stoker 
adjustments  are  maintained  and  the  draft  is  sufficient  for  the 
combustion  rates  desired.  Fuel  beds  of  more  than  6"  thick 
have  a  tendency  to  cake  and  become  irregular,  resulting  in 
increased  ash  pit  loss  and  lowered  efficiency. 

The  large  amount  of  heavy  hydro  carbons  make  this  coal 
difficult  to  burn  smokelessly  and  overfeed  stokers  require  fur- 
nace designs  combining  a  considerable  amount  of  brick  work 
and  a  long  flame  travel.  The  addition  of  brick  work  to  a 
furnace,  especially  in  the  form  of  arches,  increases  the  furnace 


COMBUSTION  CHARACTERISTICS  OF  COAL  111 

temperature,  and  when  this  type  of  stoker  is  employed  for 
burning  Pittsburgh  coal  at  high  rates  of  combustion,  clinker 
formations  will  become  troublesome  unless  the  arrangement  of 
brickwork  is  carefully  considered. 

Combustion  rates  of  from  30  to  35  Ibs.  per  sq.  ft.  of  grate 
surface  per  hour  can  be  secured  for  considerable  periods  of 
time  and  peak  load  combustion  rates  of  from  40  to  42  Ibs.  can 
be  maintained  for  two  or  three  hours,  with  a  furnace  draft  of 
from  5"  to  6". 

The  underfeed  type  has  come  into  extensive  use  in  all  dis- 
tricts where  Pittsburgh  coal  is  burned.  The  feeding  action  of 
the  stoker  is  sufficient  to  keep  the  fuel  bed  broken  up  and  in  a 
uniform  condition  for  any  thickness  of  fuel  bed  which  may  be 
carried  on  this  type  of  stoker.  The  thick  mass  of  burning  fuel 
provides  the  necessary  condition  for  the  distillation  and  com- 
bustion of  the  hydro  carbons  with  the  consequent  elimination 
of  smoke  without  the  use  of  firebrick  arches.  While  the  per- 
centage of  ash  is  higher  than  in  Eastern  coal,  requiring  more 
frequent  dumping  of  refuse,  this  feature  is  satisfactorily  taken 
care  of  with  the  designs  of  underfeed  stokers  now  used  and  is 
not  considered  objectionable  in  those  plants  using  this  type  of 
stoker. 

On  account  of  the  high  volatile  content  the  combustion  is 
not  so  complete  in  the  fuel  bed  itself  as  in  the  case  of  Eastern 
coal  and  a  large  combustion  space  between  the  fuel  bed  and 
the  boiler  heating  surface  must  be  provided  varying  in  size 
according  to  the  nature  of  the  coal  and  the  combustion  rates 
which  are  desired. 

Combustion  rates  of  from  40  to  45  Ibs.  per  sq.  ft.  of  grate 
surface  per  hour  can  be  maintained  for  long  periods  of  time 
and  from  60  to  65  Ibs.  per  sq.  ft.  for  a  peak  load  of  two  or  three 
hours. 

Chain  grate  stokers  handle  most  Pittsburgh  coal  satisfac- 
torily in  spite  of  its  slight  tendency  to  cake,  although  there  are 
some  coals  in  this  district  having  strong  caking  tendencies  for 
which  the  chain  grate  is  not  suitable.  Heavy  fuel  beds  have 
a  tendency  to  produce  uneven  fires  on  account  of  the  slight 
caking  tendency,  but  if  the  fuel  is  fed  not  more  than  five  or 
six  inches  in  thickness  this  tendency  is  not  great  and  uniform 


112  MECHANICAL  STOKERS 

fires  can  be  maintained  over  the  entire  grate  surface.  On 
account  of  the  relatively  high  percentage  of  fixed  carbon  it  is 
found  desirable  to  make  the  stokers  ten  feet  or  more  in  length 
in  order  to  more  completely  burn  out  the  carbon  in  the  ash. 

In  this  connection  it  should  be  noted  that  for  a  given  rate 
of  combustion  per  unit  area  of  grate  surface,  the  length  of  time 
that  any  one  particle  of  fuel  is  in  the  furnace  is  independent 
of  the  length  of  grate  surface  provided  the  same  fuel  bed  thick- 
ness is  maintained.  For  instance,  a  stoker  15'  long  burning 
coal  at  a  given  rate  per  sq.  ft.  must  travel  50%  faster  than  a 
stoker  10'  long  burning  coal  at  the  same  rate  per  unit  area,  and 
in  both  cases  the  length  of  time  a  particle  of  fuel  is  in  the 
furnace  will  be  exactly  the  same.  For  operating  reasons,  how- 
ever, it  is  necessary  that  a  grate  speed  considerably  in  excess 
of  that  actually  required  for  the  maximum  combustion  rate 
be  provided  to  enable  the  operator  to  take  care  of  a  sudden 
demand  when  fires  may  be  thin  or  completely  burned  out  at 
the  rear.  This  maximum  speed  required  for  emergency  is  prac- 
tically the  same  for  any  length  of  grate  and  it  is  apparent  that 
the  danger  of  discharging  excessive  amounts  of  combustible  to 
the  ash  pit  will  be  greater  with  a  short  stoker. 

Combustion  rates  of  from  30  to  35  Ibs.  per  sq.  ft.  of  grate 
surface  per  hour  can  be  maintained  for  long  periods  of  time, 
and  40  Ibs.  or  slightly  more  can  be  maintained  for  peak  loads 
of  two  or  three  hours.  The  draft  required  will  be  from  .5  to 
.6"  for  maximum  combustion  rates.  A  combustion  rate  as  high 
as  50  Ibs.  can  be  secured  for  short  intervals  by  running  a  thin 
fire,  but  it  will  be  done  at  the  expense  of  a  very  heavy  loss 
to  the  ash  pit. 

Michigan  Coal. — Michigan  coal  contains  from  8  to  15% 
moisture,  30  to  35%  volatile,  45  to  50%  fixed  carbon,  10  to 
18%  ash  and  from  12,000  to  12,500  B.T.U.  per  Ib.  dry.  This 
coal  is  very  free  burning  but  has  a  considerable  tendency  to 
clinker.  The  amount  available  is  relatively  small  and  it  is  used 
in  only  a  few  plants. 

Overfeed  stokers  are  used  successfully  with  this  coal.  The 
free  burning  characteristic  combined  with  the  tendency  to 
clinker  requires  that  less  agitation  be  given  the  fuel  both  for 
the  reason  that  agitation  is  not  required  to  keep  the  fuel  bed 


COMBUSTION  CHARACTERISTICS  OF  COAL  113 

porous  and  uniform,  and  because  too  much  agitation  aggravates 
the  tendency  to  clinker.  With  proper  adjustment,  however, 
stokers  of  this  type  do  not  get  into  clinker  trouble  and  on  those 
stokers  designed  for  continuous  removal  of  ash,  this  part  of 
the  mechanism  will  work  satisfactorily.  Combustion  rates  of 
from  30  to  35  Ibs.  can  be  maintained  for  long  periods,  and 
combustion  rates  of  40  Ibs.  for  periods  of  two  or  three  hours 
with  a  furnace  draft  of  .45"  to  .5",  but  clinker  troubles  develop 
at  this  rate  of  combustion. 

Underfeed  stokers  will  burn  this  coal  at  combustion  rates  of 
about  40  Ibs.  per  sq.  ft.  per  hour  long  periods  and  from  50  to 
55  Ibs.  per  sq.  ft.  for  two  or  three  hour  periods.  The  heavy 
fuel  bed  provides  the  necessary  condition  for  the  distillation 
and  combustion  of  the  hydro  carbons  and  smokeless  combus- 
tion is  secured  without  the  assistance  of  firebrick  arches.  The 
combustion  rate  is  limited  by  the  rate  at  which  refuse  can  be 
disposed  of,  as  this  coal  will  give  serious  trouble  with  clinkers 
if  combustion  rates  in  excess  of  60  Ibs.  are  maintained  for 
long  periods  of  time. 

Chain  grate  stokers  handle  this  coal  at  combustion  rates 
of  from  30  to  35  Ibs.  per  sq.  ft.  for  long  periods  and  40  Ibs.  or 
slightly  more  for  two  or  three  hour  periods  with  a  draft  of 
from  .5"  to  .6"  in  the  furnace.  The  free  burning  characteristics 
permit  of  the  desired  thickness  of  fuel  bed  without  any  tend- 
ency of  the  fuel  to  cake.  Uniform  fires  result  and  with  proper 
provision  at  the  rear  of  the  furnace,  ash  and  clinker  are 
disposed  of  without  the  necessity  for  resorting  to  manual 
labor. 

Illinois,  Indiana  and  Missouri  Coal. — The  coals  grouped 
under  this  heading  vary  widely  in  analyses  but  have  similar 
characteristics  so  far  as  their  action  in  a  furnace  is  concerned. 
These  coals  contain  from  8  to  18%  moisture,  38  to  53%  fixed 
carbon,  28  to  36%  volatile,  8  to  20%  ash,  and  from  11,000  to 
12,500  B.T.U.  dry. 

There  are  a  few  mines  in  Indiana  which  produce  coal 
having  some  caking  properties  which  would  not  properly  come 
under  this  heading.  Although  somewhat  higher  in  ash  and 
lower  in  heating  value  than  Pittsburgh  coal,  their  action  in  a 


114  MECHANICAL  STOKERS 

furnace  is  much  more  like  that  of  the  average  Pittsburgh  coal 
than  it  is  of  the  other  coals  of  Indiana  and  Southern  Illinois. 

On  account  of  their  free  burning  nature,  they  require  no 
agitation  to  prevent  caking  and  burn  best  on  the  overfeed 
type  of  stoker  with  just  sufficient  grate  motion  to  cause  a  con- 
tinuous feed.  A  greater  agitation  simply  aggravates  the 
tendency  to  clinker.  The  coals  of  this  group  containing  less 
than  15%  ash  are  handled  satisfactorily  on  the  overfeed  stoker 
and  without  serious  clinker  troubles  at  combustion  rates  up 
to  30  Ibs.  per  sq.  ft.  per  hour  for  long  periods  and  from  35 
to  40  Ibs.  for  two  or  three  hour  periods.  For  this  latter  com- 
bustion rate  a  furnace  draft  of  .45  to  .5"  is  required.  Com- 
bustion rates  up  to  45  Ibs.  can  be  maintained  for  short  periods 
but  at  the  expense  of  greatly  increased  ash  pit  loss,  the  pro- 
duction of  objectionable  smoke  and  a  considerable  amount  of 
hand  labor.  Those  coals  having  over  15%  ash  can  be  burned 
at  slightly  lesser  combustion  rates  on  account  of  the  ash 
accumulations.  Those  stokers  designed  to  continuously  remove 
the  ash  will  do  so  up  to  30  Ibs.  combustion  rate,  but  at  maximum 
combustion  rates,  hand  cleaning  must  be  resorted  to  at 
intervals. 

The  use  of  underfeed  stokers  in  plants  burning  Illinois 
coal  is  of  recent  date  although  a  number  of  large  plants  have 
used  this  stoker  with  sufficient  success  to  warrant  their  install- 
ing additional  equipment  of  the  same  kind.  The  nature  of  the 
fuel  bed  is  such  that  prompt  ignition  and  smokeless  combus- 
tion are  secured  without  the  assistance  of  firebrick  arches. 
Combustion  rates  of  from  40  to  45  Ibs.  per  sq.  ft.  can  be  main- 
tained for  long  periods  and  from  50  to  55  Ibs.  per  sq.  ft.  for 
two  or  three  hours.  At  combustion  rates  up  to  50  Ibs.  per 
sq.  ft.  the  removal  of  clinker  is  effected  without  trouble,  but 
when  combustion  rates  exceed  65  Ibs.,  the  ability  to  dispose  of 
the  refuse  becomes  the  limiting  factor.  The  use  of  water  boxes 
along  the  clinker  line  to  overcome  this  difficulty  has  proven 
satisfactory  where  combustion  rates  of  over  50  Ibs.  per  sq.  ft. 
are  to  be  carried  for  long  periods  of  time.  The  lowest  grades 
of  this  fuel  containing  from  15  to  18%  moisture  and  20%  or 
more  of  ash  ignite  very  readily  without  the  use  of  firebrick 
arches  and  contrary  to  the  opinions  of  those  who  made  some 


COMBUSTION  CHARACTERISTICS  OF  COAL  115 

of  the  first  installations  a  firebrick  arch  is  not  required  to 
stimulate  ignition. 

Chain  grate  stokers  were  brought  to  their  present  state  of 
development  in  the  districts  where  these  coals  are  commonly 
burned.  The  free  burning  characteristics  make  the  chain  grate 
suitable  and  the  method  of  disposing  of  ash  is  such  that  with 
the  proper  construction  at  the  rear  of  the  furnace  there  is  no 
trouble  disposing  of  the  refuse.  Combustion  rates  of  from 
30  to  35  Ibs.  can  be  maintained  for  long  periods  and  from  40 
to  45  Ibs.  for  peak  loads  with  a  furnace  draft  of  from  .5  to 
.55". 

The  tendency  to  design  boilers  with  a  narrow  furnace  width 
for  a  given  capacity  and  the  increasing  tendency  to  operate 
boilers  at  high  ratings  is  met  with  this  type  of  stoker  by 
increasing  the  length  of  the  grate.  The  long  grate  surface 
makes  it  possible  to  develop  a  furnace  design  comprising  a 
long  ignition  arch  which  is  necessary  for  prompt  ignition  with 
the  higher  speed  of  grate  travel  made  necessary  by  the  long 
stoker. 

Iowa  Coal. — The  coals  from  Iowa  which  are  available  for 
use  in  steam  plants  are  of  much  lower  grade  than  would  be 
apparent  from  the  analyses  usually  given.  The  analyses  are 
usually  made  on  mine  samples  and  will  run  lower  in  ash  and 
higher  in  heating  value  than  the  fuels  available  for  steam 
purposes.  This  is  true  to  only  a  small  degree  with  the  better 
grades  of  coal,  in  the  United  States,  but  Iowa  coal  occurs  in 
relatively  thin  veins  and  a  considerable  amount  of  dirt  and 
rock  find  their  way  into  the  screenings  which  are  usually  used 
in  steam  plants.  It  is  not  uncommon  to  find  actual  samples 
which  run  from  15  to  20%  moisture,  from  25  to  30%  ash  and 
not  over  8000  B.T.U.  per  Ib.  as  fired,  and  this  is  about  the 
grade  of  coal  that  should  be  considered  in  selecting  stoker 
equipment,  if  coal  is  to  be  bought  in  the  open  market. 

Overfeed  stokers  handle  this  coal  but  at  limited  combustion 
rates,  the  coal  being  very  hard  to  ignite  and  requiring  long 
arches  for  maintaining  the  high  temperatures  at  the  point 
where  the  fuel  enters  the  furnace  which  are  necessary  for 
prompt  ignition.  The  arch  construction  aggravates  the  tend- 
ency of  this  coal  to  clinker  and  considerable  trouble  from  this 


116  MECHANICAL  STOKERS 

source  develops  at  combustion  rates  of  over  20  Ibs.  per  sq.  ft. 
per  hour.  The  draft  required  is  somewhat  higher  than  would 
be  necessary  for  the  better  grades  of  free  burning  coal  because 
the  high  ash  content  and  clinkering  tendencies  produce  a  fuel 
bed  which  offers  an  unusual  amount  of  resistance  to  the  flow 
of  air,  a  draft  of  from  .35  to  A"  being  required  for  combustion 
rates  of  from  20  to  25  Ibs.  Combustion  rates  in  excess  of  25 
Ibs.  can  only  be  secured  at  the  expense  of  an  excessive  amount 
of  hand  labor  and  a  large  loss  of  fuel  to  the  ash  pit. 

The  presence  of  a  large  amount  of  readily  fusible  ash  adds 
to  the  difficulty  of  burning  this  fuel  without  an  excessive 
ash  pit  loss.  If  small  particles  of  clinker  which  apparently 
contain  no  combustible  be  broken  open,  it  will  be  found  that 
the  ash  has  fused  over  a  particle  of  partially  burned  coal 
cutting  off  the  air  supply  and  preventing  the  final  combustion 
of  such  fuel.  This  condition  is  aggravated  by  agitating  the 
fuel  bed  and  best  results  can  be  secured  when  this  agitation  is 
kept  at  a  minimum. 

Underfeed  stokers  of  the  self-cleaning  type  have  recently 
been  applied  in  a  number  of  plants  burning  Iowa  coal.  Ignition 
is  maintained  without  the  assistance  of  firebrick  arches  and 
the  nature  of  the  fuel  bed  is  such  that  arch  constructions  are 
not  necessary  for  smokeless  combustion.  The  high  ash  content 
and  tendency  to  clinker  necessitate  frequent  cleaning  of  the 
fires  and  at  high  rates  of  combustion,  considerable  manual  labor 
is  involved  unless  the  furnaces  are  provided  with  water  boxes 
or  some  other  arrangement  for  preventing  the  formation  of 
clinker  on  the  furnace  walls.  The  presence  of  a  large  amount 
of  ash  does  not  seriously  affect  the  ability  to  burn  the  fuel 
and  the  limitations  as  to  combustion  rates  are  determined  by 
the  ability  to  dispose  of  the  refuse  deposited  on  the  dump 
plates.  Combustion  rates  of  from  35  to  40  Ibs.  per  sq.  ft.  can 
be  maintained  for  long  periods  and  about  45  to  47  Ibs.  per  sq. 
ft.  can  be  burned  for  periods  of  two  or  three  hours.  The  use 
of  water  boxes  or  some  other  device  for  preventing  the  adhesion 
of  clinker  to  the  brickwork  should  be  installed  if  combustion 
rates  of  over  50  Ibs.  per  sq.  ft.  are  desired. 

Chain  grate  stokers  with  arch  construction  of  special  design 
give  good  service  on  this  grade  of  coal.  The  chief  problem 


COMBUSTION  CHARACTERISTICS  OF  COAL  117 

is  one  of  continuous  ignition  but  furnace  designs  obtaining 
this  result  have  been  developed.  There  is  a  tendency  even 
after  the  coal  has  been  ignited  on  the  surface  for  that  part 
lying  directly  on  the  grate  surface  to  ignite  very  slowly  if  at 
all,  and  improper  furnace  design  results  in  some  of  this  coal 
passing  to  the  ash  pit  almost  entirely  unburned.  Continuous 
ignition  and  the  combustion  of  the  greater  part  of  the  fixed 
carbon  can  be  maintained  at  combustion  rates  up  to  30  Ibs. 
per  sq.  ft.  with  a  draft  of  from  .35  to  .4"  in  the  furnace.  Com- 
bustion rates  in  excess  of  30  Ibs.  should  not  be  depended  on 
for  long  periods  of  time  because  it  is  difficult  to  maintain 
ignition  and  to  burn  out  the  fixed  carbon  at  higher  rates.  The 
arch  construction  necessary  for  continuous  ignition  has  a 
tendency  to  produce  clinker  but  the  action  of  the  grate  is 
such  that  this  is  continuously  discharged  to  the  ash  pit  and 
will  not  cause  trouble  in  the  furnace  if  the  walls  are  kept 
clear  of  clinker  and  the  fuel  is  not  permitted  to  pile  up  at  the 
bridgewall. 

The  forced  blast  traveling  grate  presents  attractive  pos- 
sibilities of  utilizing  low  grade  Iowa  coals.  Ignition  is 
easily  secured  because  the  air  supply  can  be  regulated  to 
maintain  the  desired  combustion  rate  and  temperature  at  the 
front  of  the  furnace  even  during  periods  of  light  load.  A 
substantially  constant  condition  can  be  maintained  in  the 
ignition  zone  for  all  ratings  and  the  length  of  fire  and  com- 
bustion rate  on  the  remainder  of  the  grate  surface,  regulated 
to  suit  the  load  requirements. 

In  some  localities  washed  Iowa  coal  is  available  which  is 
of  much  better  quality  than  that  mentioned  above  and  com- 
pares quite  favorably  with  the  high  ash  coals  of  Northern 
Illinois  although  it  should  not  be  considered  in  the  same  class 
as  the  Southern  Illinois  fuels.  It  is  especially  important  that 
Iowa  coal  should  be  of  suitable  size  as  a  large  percentage  of 
coarse  coal  retards  ignition  by  admitting  too  much  cold  air 
at  the  front  of  the  furnace.  Screenings  which  have  passed 
through  a  l1/^  to  l1/^"  screen  are  of  suitable  size  but  larger 
particles  if  present  in  large  quantities  will  seriously  effect 
the  operation. 


118  MECHANICAL  STOKERS 

East  Kentucky,  Tennessee  and  Alabama  Coal. — The  coals 
from  this  district  contain  from  30  to  37%  volatile,  from  50  to 
60%  carbon,  from  5  to  15%  ash,  and  from  13,000  to  13,500 
B.T.U.  per  Ib.  dry.  These  are  thoroughly  high  grade  and  as 
a  class  are  practically  free  burning  although  the  coal  from 
some  localities  in  this  district  has  a  considerable  tendency  to 
cake.  The  high  volatile  content  requires  that  stokers  be  set 
with  liberal  combustion  chambers  for  smokeless  operation. 

Overfeed  stokers  both  of  the  front  feed  and  side  feed 
types,  provide  sufficient  agitation  to  keep  the  fuel  bed  in  a 
uniform  and  porous  condition  and  handle  all  coals  in  this  dis- 
trict satisfactorily.  Combustion  rates  of  from  30  to  35  Ibs.  can 
be  maintained  for  long  periods  and  for  peak  loads,  40  lbs.;  may 
be  burned  for  short  intervals,  with  a  draft  of  from  .5  to  .6" 
in  the  furnace.  The  low  ash  coals  of  this  district  do  not  pro- 
duce an  ash  layer  of  sufficient  thickness  to  protect  the  grates 
and  unless  the  stokers  are  properly  handled,  considerable  burn- 
ing of  the  grate  surface  will  result.  In  general  however,  it 
may  be  said  that  an  installation  of  overfeed  stokers  will  be 
very  satisfactory  if  surrounded  with  the  proper  conditions. 

Underfeed  stokers  are  handling  this  coal  successfully.  Fire- 
brick arches  are  not  required  and  the  feeding  action  prevents 
caking  of  those  coals  which  have  a  tendency  to  do  so.  Com- 
bustion rates  of  from  45  to  50  Ibs.  per  sq.  ft.  for  long  periods 
and  from  55  to  60  Ibs.  for  peak  loads  can  be  maintained. 

The  thick  fuel  bed  provides  a  means  for  burning  the  volatile 
and  this  combustion  is  to  a  large  extent,  completed  in  the 
fuel  bed  itself.  Liberal  combustion  chambers  are  desirable, 
however,  for  smokless  operation  and  best  efficiency  at  high 
combustion  rates.  The  low  ash  content  and  the  comparatively 
high  fusing  temperature  of  the  ash  make  it  possible  to  operate 
at  high  ratings  without  clinker  trouble. 

Chain  grate  stokers  handle  some  coals  from  this  district 
satisfactorily  but  are  not  considered  suitable  for  others.  The 
absence  of  agitation  allows  the  caking  of  those  coals  which  have 
a  tendency  do  so  with  the  result  that  fuel  beds  become 
irregular,  allowing  excess  air  to  enter  through  the  rear  of  the 
grate  and  the  discharge  of  an  undue  amount  of  coke  to  the 
ash  pit.  Those  coals  having  very  low  ash  content  do  not 


COMBUSTION  CHARACTERISTICS  OF  COAL  119 

sufficiently  protect  the  grate  surface  in  the  rear  of  the  furnace 
and  maintenance  will  be  above  the  normal  amount  for  this 
type  of  stoker  unless  a  cooling  effect  is  secured  by  permitting 
the  entrance  of  a  considerable  excess  of  air  in  the  rear  portion 
of  the  grate  surface.  The  remaining  fuels  can  be  handled 
successfully  at  combustion  rates  of  from  30  to  35  Ibs.  per  sq. 
ft.  for  long  periods  and  about  40  Ibs.  for  peak  loads  with  a 
draft  of  from  .5  to  .6"  in  the  furnace. 

Those  chain  grates  equipped  with  mechanism  for  agitating 
the  fuel  bed  during  the  early  stages  of  combustion  can  burn 
the  caking  coals  of  this  district  at  about  the  same  rate  as 
given  above  for  the  standard  type  of  chain  grate. 

Texas,  Oklahoma  and  Arkansas  Coal. — There  are  three 
distinct  coals  available  in  this  district.  First,  semi-anthra- 
cite containing  from  10  to  16%  volatile,  1  to  2%  mois- 
ture, 77%  fixed  carbon  and  8  to  18%  ash.  Second,  bituminous 
coal  having  from  30  to  35%  volatile,  30  to  40%  fixed  carbon, 
and  10  to  20%  ash,  although  these  coals  as  actually  delivered 
to  the  plant  may  contain  as  high  as  30%  ash. 

Third,  lignite  containing  from  20  to  35%  moisture,  30  to 
40%  volatile,  fixed  carbon  30  to  35%,  and  ash  10  to  15%. 

On  account  of  the  wide  variations  in  fuels  from  adjacent 
localities,  it  is  advisable  to  secure  actual  analyses  of  coal  as 
delivered  rather  than  to  depend  upon  average  analyses.  The 
methods  of  mining  and  preparation  will  control  the  ash  content 
and  may  have  an  important  bearing  on  the  manner  in  which 
these  fuels  will  burn.  There  are  cases  where  mine  samples 
may  show  from  15  to  18%  ash  but  where  fuels  available  for 
steam  purposes  will  run  as  high  as  35  to  40%  ash.  It  is 
apparent  that  apparatus  designed  to  burn  the  former  might 
be  entirely  unsuited  to  the  fuel  as  actually  delivered. 

The  semi-anthracite  coal  being  comparatively  low  in  volatile 
and  high  in  ash  is  extremely  difficult  to  ignite.  Overfeed 
stokers  require  special  arch  constructions  to  maintain  ignition, 
the  high  fixed  carbon  content  results  in  a  hot  fire  after  ignition 
has  been  thoroughly  established  and  the  large  amount  of  brick- 
work necessary  for  ignition  tends  to  increase  furnace  tem- 
peratures to  such  an  extent  that  clinker  troubles  result  from 
the  fusing  of  the  ash.  Combustion  rates  up  to  25  Ibs.  can  be 


120  MECHANICAL  STOKERS 

maintained  with  A"  furnace  draft  but  this  combustion  rate 
cannot  be  exceeded  without  considerable  clinker  trouble  and 
manual  labor. 

Underfeed  stokers  ignite  this  fuel  without  the  assistance 
of  firebrick  arches  and  can  maintain  combustion  rates  of  38 
to  40  Ibs.  per  sq.  ft.  This  can  be  increased  to  50  Ibs.  per  sq. 
ft.  for  peak  loads  but  at  high  combustion  rates  the  clinker 
trouble  becomes  objectionable. 

This  coal  being  of  a  free  burning  nature  can  be  handled 
satisfactorily  on  chain  grate  stokers  with  special  arch  con- 
struction suitable  for  maintaining  continuous  ignition.  Com- 
bustion rates  up  to  30  Ibs.  per  sq.  ft.  can  be  maintained  for 
long  periods  with  a  furnace  draft  of  about  A"  but  attempts 
at  high  combustion  rates  usually  result  in  delayed  ignition. 
The  low  grade  Texas  lignite  is  not  used  to  a  large  extent  for 
steam  purposes  and  the  adaptability  of  the  different  types  of 
stoker  to  this  coal  can  only  be  fully  determined  by  further 
experiments.  The  principal  difficulty  encountered  is  that  of 
thoroughly  driving  off  the  moisture  and  burning  the  fuel  in 
the  same  furnace.  Satisfactory  results  could  unquestionably 
be  secured  by  a  pre-drying  process  independent  of  the  fuel  burn- 
ing furnace  but  the  expense  of  this  construction  added  to  the 
cost  of  the  standard  stoker  equipment  is  not  justified  under 
present  conditions.  Whether  a  pre-drying  process  or  a  special 
furnace  and  stoker  construction  would  be  preferable  can  only 
be  determined  by  careful  experiments  which  will  only  be 
carried  out  when  the  fuel  is  more  attractive  from  a  stand- 
point of  comparative  cost.  With  the  present  development  of 
the  art  it  seems  that  the  forced  draft  traveling  grate  offers 
the  best  solution  of  this  problem.  The  indications  are  that  if 
this  fuel  were  crushed  so  that  no  particles  would  pass  over  a 
V  screen,  that  it  could  be  successfully  handled  on  the  forced 
draft  traveling  grate. 

The  bituminous  coals  of  Texas  are  not  important  from  a 
commercial  standpoint  because  they  cover  an  area  which  is 
located  a  considerable  distance  from  any  large  fuel  burning 
localities.  The  coal  as  delivered  for  steam  purposes  often  con- 
tain 30%,  or  more  of  ash,  is  difficult  to  ignite  and  clinkers 
very  badly.  It  has  been  successfully  handled  only  on  chain 


COMBUSTION  CHARACTERISTICS  OF  COAL  121 

grate  stokers  built  in  length  greater  than  ordinarily  used  and 
with  special  arch  construction.  This  coal  is  free-burning,  has 
no  tendency  to  cake  but  will  clinker  with  the  slightest  agita- 
tion, forming  a  sticky  mass  when  hot  and  a  hard  brittle 
clinker  when  cold.  Combustion  rates  of  25  Ibs.  per  sq.  ft.  can 
be  maintained  but  at  higher  rates  there  is  danger  of  delayed 
ignition  or  the  piling  up  of  the  fuel  at  the  rear  of  the  furnace 
which  will  immediately  produce  objectionable  clinkers. 

Colorado  Coals. — Colorado  produces  a  greater  variety  of 
coals  than  can  be  found  in  any  other  similar  area  in  the  United 
States.  In  the  Northwestern  part  of  the  state,  a  fair  grade 
of  anthracite  is  to  be  had.  In  the  district  north  of  Denver 
there  are  large  deposits  of  lignite  of  considerable  commercial 
value  also  lignite  deposits  in  the  vicinity  of  Colorado  Springs. 
The  Western  part  of  the  state  produces  a  high  volatile  free 
burning  coal  similar  to  the  best  grades  of  Southern  Illinois 
while  in  the  southern  part  of  the  state  a  high  volatile  caking 
coal  similar  in  many  respects  to  the  high  volatile  coking  coals 
of  West  Virginia  is  found. 

Colorado  Anthracite. — This  coal  is  of  small  importance 
because  it  is  located  at  a  considerable  distance  from  any  large 
fuel  burning  districts  and  is  not  used  to  any  great  extent 
for  steam  purposes.  It  can  be  burned  in  the  same  manner  as 
the  anthracite  coals  of  Eastern  Pennsylvania  which  will  be 
referred  to  later. 

Lignite. — Colorado  lignite  is  of  greater  importance  com- 
mercially than  any  other  lignite  deposit  in  the  United  States. 
Being  located  in  the  vicinity  of  Denver  there  is  a  market  for 
large  quantities  for  steam  purposes.  It  is  of  much  better 
quality  than  the  Texas  lignite  and  contains  from  15  to  25% 
moisture,  8  to  15%  ash,  30  to  35%  volatile,  and  from  10,000  to 
12,000  B.T.U.  dry. 

Overfeed  stokers  ignite  this  coal  satisfactorily  when  pro- 
vided with  suitable  arch  constructions  and  give  satisfactory 
results  at  combustion  rates  of  from  20  to  25  Ibs.  per  sq.  ft. 
At  higher  combustion  rates  ignition  becomes  uncertain  and 
clinker  troubles  develop. 

This  fuel  is  very  light  after  thoroughly  dried  and  high 
furnace  draft  cannot  be  employed  with  the  fuel  bed  of  the 


122  MECHANICAL  STOKERS 

thickness  which  can  be  carried  on  this  type  of  stoker.  There 
is  also  considerable  difficulty  experienced  due  to  avalanching 
and  it  requires  careful  and  skillful  attention  to  secure  and 
maintain  good  fuel  bed  conditions. 

Underfeed  stokers  ignite  this  coal  continuously  and  the 
heavy  bed  at  the  point  of  fuel  introduction  provides  the  neces- 
sary conditions  for  the  complete  driving  off  of  the  moisture 
before  the  fuel  enters  the  active  combustion  zone.  When 
thoroughly  dried  this  fuel  can  be  burned  very  rapidly  and 
while  it  has  a  tendency  to  clinker,  the  clinker  formation  is  not 
sufficiently  serious  to  prevent  the  satisfactory  operation  and 
cleaning  of  the  furnace. 

A  firebrick  arch  is  not  required  for  the  purpose  of  ignition 
under  ordinary  conditions  although  it  would  prove  desirable 
in  case  it  was  desired  to  maintain  high  combustion  rates  for 
several  hours  at  a  time.  For  ordinary  peak  load  service,  it 
is  found  that  this  lignite  gives  practically  the  same  results 
so  far  as  the  ability  to  meet  sudden  demands  is  concerned  as 
the  better  grades  of  bituminous  coal.  There  is  at  all  times  a 
large  quantity  of  fuel  in  the  furnace  that  has  been  thoroughly 
dried  out  and  ignited  and  in  this  condition  it  makes  a  fuel 
bed  which  responds  readily  to  an  increase  in  air  supply  and 
makes  it  comparatively  easy  to  meet  a  sudden  demand  for 
steam.  Combustion  rates  of  from  30  to  40  Ibs.  per  sq.  ft.  can 
be  maintained  for  long  periods  and  from  50  to  55  Ibs.  for  short 
peaks. 

Due  to  the  lightness  of  this  fuel  the  high  air  velocities 
through  the  upper  part  of  the  fuel  bed  carry  considerable 
quantities  of  fine  particles  up  into  the  combustion  chamber 
which  are  deposited  at  the  rear  of  the  furnace.  This  con- 
dition produces  a  tendency  for  the  fuel  bed  to  drift  and  become 
heavy  over  the  dump  grates.  In  order  to  overcome  this  dif- 
ficulty, it  is  desirable  to  make  changes  from  the  standard 
method  of  introducing  the  air  so  that  the  air  supply  to  the 
rear  sections  can  be  controlled  independent  of  the  air  supply 
to  the  front  sections.  This  tends  to  reduce  the  tendency  to 
drift  and  at  the  same  time,  makes  it  possible  to  burn  out  com- 
pletely such  fuel  as  does  lodge  over  the  rear  sections. 

Chain  grate  stokers  are  used  successfully  on  the  various 


COMBUSTION  CHARACTERISTICS  OF  COAL  123 

grades  of  Colorado  lignite  the  only  requirement  so  far  as  the 
fuel  is  concerned  being  that  it  must  not  be  too  coarse.  If  a 
considerable  percentage  will  pass  over  a  iy2"  screen,  it  is  too 
coarse  for  best  results  and  should  be  reduced  to  finer  size. 
Suitable  furnace  designs  have  been  developed  to  burn  this 
coal  at  combustion  rates  up  to  45  Ibs.  per  sq.  ft.  per  hour  and 
this  combustion  rate  can  be  increased  with  special  designs. 
Surface  ignition  occurs  promptly  and  the  drying  process  which 
must  precede  ignition  goes  on  rapidly  enough  to  permit  ignition 
to  the  full  depth  in  a  travel  of  two  or  three  feet. 

While  lignite  has  a  very  large  percentage  of  volatile  based 
on  dry  fuel  there  is  little  or  no  trouble  securing  smokeless 
operation  for  the  reason  that  the  furnace  constructions  required 
for  satisfactory  ignition  provide  the  necessary  conditions  for 
smokeless  combustion. 

Colorado  Bituminous. — The  high  volatile  bituminous  coals 
both  caking  and  noncaking,  burn  satisfactorily  on  overfeed 
stokers.  The  action  of  the  grate  surface  is  sufficient  to  keep 
the  fuel  bed  in  a  uniform  and  porous  condition  without  agitat- 
ing the  fuel  a  sufficient  amount  to  produce  clinkers.  Those 
stokers  provided  with  continuous  cleaning  mechanism  will 
remove  ash  continuously  except  under  most  severe  conditions 
of  overloading  when  it  may  be  necessary  to  supplement  the 
action  of  the  cleaning  mechanism  by  a  certain  amount  of  hand 
labor.  Combustion  rates  up  to  30  Ibs.  per  sq.  ft.  can  be  main- 
tained for  long  periods,  and  from  35  to  40  Ibs.  for  short  inter- 
vals, with  a  draft  in  the  furnace  of  from  .5  to  .55". 

Underfeed  stokers  will  burn  all  grades  of  Colorado  bitumi- 
nous fuels.  Firebrick  arches  are  not  required  to  maintain 
ignition  but  on  account  of  the  very  high  volatile  content  of 
many  of  these  fuels  a  large  combustion  chamber  is  desirable 
for  best  results.  The  feeding  action  of  the  stoker  is  sufficient 
to  break  up  the  fuel  bed  in  case  there  is  a  tendency  to  cake 
and  on  account  of  the  relatively  low  sulphur  content,  clinker 
troubles  do  not  occur  under  proper  conditions  of  operation. 
Combustion  rates  of  from  40  to  50  Ibs.  per  sq.  ft.  can  be  main- 
tained for  long  periods,  and  from  55  to  65  Ibs.  per  sq.  ft.  for 
peak  loads. 

Chain  grate  stokers  handle  most  of  these  coals  satisfactorily 


124  MECHANICAL  STOKERS 

although  there  are  a  few  which  have  sufficient  caking  tenden- 
cies to  make  the  chain  grate  undesirable.  The  high  volatile 
content  makes  this  coal  very  easy  to  ignite  and  the  arch  con- 
struction is  therefore  determined  more  from  the  standpoint 
of  securing  smokeless  operation  than  for  providing  suitable 
ignition.  Those  fuels  which  are  free  burning  will  produce 
even  and  uniform  fires  and  good  operating  results  can 
be  secured.  Combustion  rates  of  from  35  to  40  Ibs.  per  sq.  ft. 
can  be  maintained  for  long  periods  and  45  Ibs.  or  slightly  more 
for  short  periods,  with  a  furnace  draft  of  from  .5  to  .6". 

Washington  and  Oregon  Coal. — The  coal  deposits  of  Wash- 
ington cover  large  areas  and  contain  an  immense  amount  of 
fairly  high  grade  fuel.  They  have  not  been  highly  developed, 
however,  largely  on  account  of  the  large  quantity  of  fuel  oil 
available  at  low  prices.  The  rapidly  changing  conditions  of 
the  oil  market,  however,  will  result  in  a  large  increase  in  the 
demand  for  fuel  in  those  districts  which  can  be  supplied  from 
Washington  fields.  In  analysis  and  characteristics,  this  coal 
is  quite  similar  to  Illinois  fuels  and  can  be  burned  with  prac- 
tically the  same  stoking  equipment  and  with  the  same  general 
results. 

Wyoming  Coal. — There  are  a  number  of  deposits  of  coal 
in  Wyoming  but  on  account  of  their  distance  from  large  fuel 
burning  localities  their  development  has  not  been  rapid.  They 
compare  in  analysis  and  characteristics  with  the  free  burning 
middle  Western  coals,  and  can  be  burned  on  the  same  stokers 
with  practically  the  same  results. 

Dakota  Lignite. — There  are  large  deposits  of  lignite  in 
Dakota  of  commercial  value  being  only  of  slightly  lower 
quality  than  those  of  Colorado.  Until  recently,  they  had 
received  but  little  attention  but  present  developments  in  the 
fuel  situation  have  resulted  in  greatly  increased  importance 
of  these  fuels.  Until  recently  Minnesota  and  the  Dakotas  have 
been  burning  Pittsburgh  and  West  Virginia  coals.  These  coals 
have  been  able  to  reach  this  district  because  they  furnish  a 
suitable  return  cargo  for  the  ore  boats  from  the  Lake  Superior 
Iron  district.  Transportation  conditions  have  become  less 
favorable,  however,  and  Illinois  coal  has  been  finding  a  ready 
market  in  this  district.  It  is  necessary  to  provide  for  the 


COMBUSTION  CHARACTERISTICS  OF  COAL  125 

winter's  supply  during  the  summer  and  fall  to  insure  against 
the  delays  in  transportation  and  the  Illinois  coal  is  not  entirely 
suitable  for  storage  on  account  of  the  tendency  to  spontaneous 
ignition. 

The  Dakota  lignites  are  not  a  great  distance  from  the 
Dakota  and  Minnesota  markets  and  if  they  can  be  satisfactorily 
burned  will  offer  a  solution  of  the  fuel  problem  in  this  dis- 
trict. While  few  plants  are  burning  this  fuel  regularly,  con- 
siderable experimental  work  has  been  done.  Special  hand 
fired  furnaces  have  been  developed  which  are  suitable  for 
small  boilers  and  suitable  furnaces  have  been  devised  for  burn- 
ing this  coal  on  stokers. 

If  the  lignite  is  crushed  to  small  size  it  can  be  burned  on 
chain  grate  stokers  having  special  arch  constructions  and  the 
results  secured  give  promise  of  commercial  success.  The  most 
satisfactory  results  have  been  secured  from  underfeed  stokers 
equipped  with  a  short  firebrick  arch  over  the  front  of  the 
furnace  to  increase  the  furnace  temperatures  at  this  point 
and  increase  the  rate  at  which  the  fuel  may  be  fired  and 
completely  ignited.  Combustion  rates  of  from  30  to  40  Ibs. 
per  sq.  ft.  can  be  maintained  continuously  and  from  55  to  60 
Ibs.  for  peak  loads. 

Further  developments  in  the  application  of  stokers  to  the 
burning  of  this  fuel  will  probably  be  along  the  line  of  the 
forced  draft  traveling  grate. 

The  principal  difficulty  is  that  of  rapid  drying  and  ignition 
of  the  fuel  entering  the  furnace.  If  a  high  furnace  draft  is 
employed,  special  provision  must  be  made  to  prevent  air 
leakage  around  the  front  of  the  furnace  because  this  leakage 
seriously  retards  ignition.  The  forced  draft  traveling  grate 
can  be  operated  with  atmospheric  pressure  over  the  fire  thus 
eliminating  this  air  leakage  and  providing  better  furnace  con- 
ditions for  prompt  ignition.  The  positive  control  of  air  to 
different  portions  of  the  fuel  bed  also  assist  in  securing  the 
conditions  necessary  for  the  successful  burning  of  such  fuel. 

Anthracite  Culm. — Large  quantities  of  anthracite  culm  are 
available  in  the  districts  adjoining  the  anthracite  fields.  This 
culm  has  for  years  been  burned  with  a  fair  degree  of  success 
on  specially  constructed  hand  fired  grates  equipped  with  force 


126  MECHANICAL  STOKERS 

draft  but  the  successful  application  of  stokers  is  of  more  recent 
date. 

Underfeed  stokers  have  been  used  successfully  especially 
when  the  culm  is  mixed  with  a  certain  percentage  of  bituminous 
coal.  Both  the  capacity  and  the  efficiency  which  can  be 
secured,  decrease  with  the  increase  in  percentage  of  culm  in 
the  mixture,  and  at  average  prices  for  the  two  fuels  the  cost 
of  generating  steam  will  decrease  slightly  by  the  use  of  culm 
until  the  mixture  contains  from  40  to  50%  culm.  Increasing 
the  percentage  of  culm  above  this  point  so  effects  both  capacity 
and  efficiency  that  the  cost  of  steam  is  increased.  For  the 
suitable  burning  of  such  fuels  the  mixture  must  be  very 
thorough.  It  is  difficult  with  the  coal  handling  equipment 
usually  installed  to  get  a  suitable  mixture  and  even  then  there 
is  a  tendency  for  the  two  fuels  to  separate  in  the  coal  bunkers. 
In  order  to  make  this  method  of  burning  anthracite  suc- 
cessful, a  specially  designed  coal  handling  apparatus  is 
required  which  will  insure  a  uniform  mixture  entering  the 
furnace.  The  cost  of  this  arrangement  and  the  added  difficul- 
ties in  burning  mixed  fuels  makes  the  whole  scheme  of  rather 
doubtful  commercial  value. 

Overfeed  stokers  have  been  used  in  a  number  of  cases  on 
straight  anthracite  but  not  with  complete  success.  This  coal 
is  very  fluid  and  has  a  decided  tendency  to  avalanche  on 
inclined  types  of  stokers.  It  also  clinkers  badly  when  agitated 
and  the  maintenance  of  a  uniform  fuel  bed  is  rendered  quite 
difficult,  both  on  account  of  clinker  formations  and  the  tend- 
ency to  avalanche.  Combustion  rates  of  20  Ibs.  or  slightly 
more  can  be  secured  with  a  furnace  draft  of  from  .4  to  .5" 
and  better  results  can  be  secured  by  the  application  of  forced 
draft.  The  ignition  is  more  prompt  and  the  combustion  rates 
can  be  slightly  increased. 

The  forced  draft  traveling  grate  has  been  most  successful 
in  handling  this  fuel.  On  account  of  its  very  small  size, 
specially  designed  grate  surfaces  having  small  air  spaces  are 
required  to  prevent  the  sifting  of  fuel  through  the  grates. 
Stokers  of  this  type  are  made  in  lengths  of  from  12  to  17  ft. 
and  are  divided  into  compartments  for  the  control  of  the  air 
pressure  to  different  sections  of  the  fuel  bed.  By  the  use  of 


COMBUSTION  CHARACTERISTICS  OF  COAL  127 

special  arch  constructions,  prompt  ignition  is  secured  which 
can  be  maintained  at  combustion  rates  up  to  50  Ibs.  per  sq.  ft. 
Combustion  rates  up  to  40  Ibs.  can  be  secured  for  long  periods 
of  time  and  since  the  air  supply  is  under  control,  uniform 
fuel  beds  can  be  maintained.  On  account  of  the  lengths  in 
which  this  type  of  stoker  can  be  built,  a  large  ratio  of  grate 
surface  to  heating  surface  is  secured  and  high  overload  capaci- 
ties can  be  developed. 

Coke  Breeze. — On  account  of  the  brittle  nature  of  coke, 
large  quantities  of  fine  particles  are  broken  off  in  handling 
both  at  the  coke  ovens  and  at  the  furnaces  where  the  coke 
is  burned.  At  such  places  large  piles  of  coke  breeze  are  nearly 
always  in  evidence  and  if  no  space  is  available  for  piling  up 
this  material  it  is  often  hauled  away  to  a  convenient  place 
and  dumped. 

Many  attempts  have  been  made  to  burn  this  breeze  which 
contains  from  10,500  to  11,500  B.T.U.  per  Ib.  but  on  account 
of  its  low  volatile  and  high  ash  contents  it  is  difficult  to  ignite 
and  produces  large  amounts  of  clinker.  Being  very  fine,  it 
offers  considerable  resistance  to  the  flow  of  air  and  requires 
forced  draft  for  its  combustion.  Hand  fired  grates  equipped 
with  forced  draft  have  been  used  to  a  certain  extent  but  the 
fact  that  it  has  been  quite  common  practice  at  some  plants 
to  buy  coal  for  their  boiler  plants  and  throw  away  the  coke 
breeze  would  indicate  the  results  have  not  been  satisfactory. 
The  principal  difficulty  has  been  the  rapid  clinkering  of  the 
fuel  bed  and  the  large  amount  of  manual  labor  required  to 
keep  the  fires  in  such  condition  that  reasonable  combustion 
rates  could  be  maintained. 

In  a  few  cases  overfeed  stokers  have  been  used  by  screen- 
ing the  coke  breeze  and  using  the  larger  particles.  The  igni- 
tion is  slow  and  rather  uncertain  and  clinker  formations  are 
very  objectionable.  The  amount  of  labor  involved  is  high  and 
the  rating  which  can  be  secured  are  not  high  enough  to  warrant 
the  installation  of  a  sufficient  number  of  boilers  to  carry  the 
load  in  this  manner. 

Underfeed  stokers  will  ignite  the  coke  breeze  without  the 
assistance  of  firebrick  arches  and  it  can  be  burned  more  satis- 
factorily than  on  the  overfeed  type.  Clinker  formations  are 


128  MECHANICAL  STOKERS 

serious,  however,  and  not  only  limit  the  fuel  burning  capacity 
but  involve  a  great  amount  of  hand  labor.  Coke  breeze  is 
very  abrasive  and  the  forcing  of  this  material  through  the 
throats  and  retorts  of  this  type  of  stoker  results  in  very  high 
maintenance. 

Traveling  grates  of  special  design  handle  this  fuel  much 
more  successfully  than  it  can  be  handled  in  any  other  manner. 
On  account  of  the  large  percentage  of  very  fine  particles  the 
air  space  must  be  very  finely  divided  to  prevent  sifting  and  on 
account  of  the  dense  fuel  bed,  forced  draft  must  be  employed 
to  supply  the  necessary  air  for  combustion.  The  low  per- 
centage of  volatile  makes  this  fuel  difficult  to  ignite  but  arch 
constructions  have  been  developed  which  do  this  satisfactorily. 
Since  there  is  no  relative  movement  between  the  fuel  and  the 
fuel  supporting  surface,  the  abrasion  is  eliminated  and  there 
is  no  agitation  of  the  fuel  during  combustion  to  aggravate 
clinker  formations.  The  quality  of  the  coke  breeze  will  vary 
both  on  account  of  the  quality  of  coal  from  which  the  coke  is 
made  and  on  account  of  the  amount  of  dirt  gathered  up  with 
the  coke  breeze.  The  ash  content  in  some  cases  runs  from  20 
to  25%  but  this  fuel  can  be  burned  satisfactorily  at  combustion 
rates  of  from  25  to  40  Ibs.  per  sq.  ft.  The  application  of  this 
stoker  has  made  it  possible  in  a  number  of  plants  to  discon- 
tinue the  use  of  coal  entirely  and  at  the  same  time  to  save 
the  expense  of  disposing  of  the  coke  breeze  to  a  suitable  dump. 

Bone  Goal. — In  coal  mining  operations,  it  is  necessary  to 
remove  a  certain  amount  of  material  containing  a  large  per- 
centage of  coal  but  not  of  marketable  grade.  A  certain  amount 
of  this  bone  can  be  disposed  of  underground  but  the  balance 
must  be  hoisted  to  the  surface  and  carried  to  a  suitable  dump. 
In  some  localities  the  bone  will  have  as  high  as  from  11,000 
to  11,500  B.T.U.  per  Ib.  and  from  20  to  25%  ash.  In  other 
localities,  the  ash  content  will  run  as  high  as  35%  and  the 
B.T.U.  from  eight  to  nine  thousand.  Millions  of  tons  of  this 
material  as  piled  up  around  the  mines  and  until  recently,  no 
attempt  has  been  made  to  utilize  it  as  a  fuel. 

The  forced  blast  traveling  grate  will  burn  this  coal  satis- 
factorily and  thereby  save  not  only  the  marketable  fuel  being 
burned  at  many  such  plants  but  also  solve  the  problem  of  dis- 
posing of  this  material. 


CHAPTER  V 
DRAFT 

It  has  already  been  pointed  out  that  the  majority  of  stoker 
troubles  have  been  due  to  faulty  application  rather  than  to 
defects  in  the  stokers  themselves.  Faulty  application,  in  turn, 
often  resolves  itself  into  a  question  of  draft  and  this  one  factor 
is  responsible  for  more  of  the  troubles  encountered  with 
mechanical  stokers  than  all  other  factors  combined.  On  the 
other  hand,  if  the  draft  conditions  are  good,  many  other 
factors  which  would  cause  serious  trouble  in  connection  with 
unfavorable  draft  conditions  can  be  successfully  handled  and 
the  engineer  who  makes  sure  that  a  proposed  installation  will 
have  ample  draft  has  every  prospect  of  making  a  successful 
installation  in  spite  of  some  mistakes  which  may  be  made. 

The  increasing  tendency  to  operate  boilers  at  high  over- 
loads has  introduced  another  important  factor  into  the  draft 
problem.  The  high  ratings  often  require  special  arrangements 
of  boiler  baffles  introducing  an  element  of  uncertainty  as  to 
the  draft  losses  at  high  ratings  and  unless  ample  provision 
for  draft  is  made,  contemplated  ratings  cannot  be  developed. 

Draft,  as  the  term  is  commonly  used,  refers  to  the  difference 
in  pressure  between  the  point  where  the  draft  reading  is  taken 
and  the  atmosphere.  For  instance,  if  there  is  said  to  be  a  draft 
of  one  inch  of  water  at  a  given  point,  the  pressure  at  that  point 
differs  from  the  surrounding  atmospheric  pressure  by  the 
amount  necessary  to  support  a  vertical  column  of  water  one 
inch  high.  The  draft  may  represent  a  pressure  either  above 
or  below  that  of  the  surrounding  atmosphere  but  as  the  term 
is  commonly  used  by  engineers  draft  represents  a  certain  pres- 
sure below  atmospheric  pressure  or  in  other  words,  there  is  a 
suction  at  the  point  indicated.  In  case  the  pressure  is  greater 
than  that  of  the  atmosphere,  it  is  usually  termed  "pressure" 

129 


130  MECHANICAL  STOKERS 

or  "plenum."  In  the  case  of  a  forced  draft  installation, 
where  the  air  is  supplied  to  the  under  side  of  the  grates  at 
more  than  atmospheric  pressure,  the  amount  which  this  exceeds 
the  pressure  of  the  atmosphere  is  expressed  as  "pressure"  in 
inches  of  water,  and  the  pressure  of  the  gases  passing  through 
the  boiler  will  be  at  less  than  atmospheric  pressure,  the  amount 
being  expressed  as  "draft"  in  inches  of  water.  Since  this  is 
the  common  usage,  the  term  "pressure"  will  hereafter  be 
used  to  indicate  pressures  above  that  of  the  surrounding 
atmosphere  and  "draft"  to  indicate  pressures  below  that  of 
the  atmosphere. 

In  practice,  draft  is  produced  in  three  ways — first,  "natural 
draft"  which  depends  upon  the  difference  in  weight  of  a 
vertical  column  of  hot  gases  in  a  chimney  and  that  of  a  cor- 
responding height  of  the  surrounding  air;  second,  "induced 
draft, ' '  which  is  commonly  produced  by  means  of  a  fan  taking 
the  gases  to  be  handled  from  a  suitable  flue  at  less  than 
atmospheric  pressure  and  discharging  them  against  atmospheric 
pressure;  third,  "forced  draft,"  where  a  fan  or  blower  takes 
air  at  atmospheric  pressure  and  discharges  it  to  a  duct  or 
wind  box  at  a  pressure  above  •  that  of  the  atmosphere.  In 
practice,  either  natural  draft  or  induced  draft  may  be  used 
singly  but  when  forced  draft  is  employed,  either  natural  or 
induced  draft  is  required  to  remove  the  products  of  combustion 
from  the  furnace. 

Natural  draft. — Since  gases  expand  as  the  temperature  is 
increased,  provided  the  pressure  is  maintained  substantially 
constant,  the  weight  per  unit  of  volume  will  decrease.  In  the 
case  of  a  vertical  flue  or  chimney  containing  hot  gases,  the 
weight  of  these  gases  will  be  less  than  that  of  an  equal  column 
of  cold  air  and  the  pressure  at  the  bottom  of  the  stack  will 
therefore  be  less  than  the  atmospheric  pressure  at  the  same 
elevation.  There  would,  therefore,  be  a  tendency  for  the  cold 
air  to  flow  to  the  inside  of  the  chimney  provided  a  suitable 
opening  were  provided  at  or  near  its  base.  If  some  means  be 
provided  for  heating  the  air  as  it  enters  the  base  of  the 
chimney,  a  draft  at  this  point  will  be  maintained  which  will 
cause  a  continual  flow. 

In  most  draft  calculations  it  is  assumed  that  the  chimney 


DRAFT  131 

gases  have  the  same  density  as  air  at  the  same  temperature 
and  pressure.  A  column  of  air  one  foot  square  and  one  hun- 
dred feet  high  will  weigh  7.78  Ibs.  at  a  temperature  of  50°  F. 
while  a  column  of  the  same  dimensions  but  having  a  tempera- 
ture of  500°  F.  would  weigh  only  4.13  Ibs.,  a  difference  of  3.65 
Ibs.  A  chimney  100'  high  and  containing  gases  at  500°  F. 
with  the  outside  air  at  50°  F.  would  therefore  exert  at  its 
base  a  pressure  of  3.65  Ibs.  per  sq.  ft.  or  .02535  Ib.  per  sq. 
inch,  less  than  atmospheric  pressure. 

One  cubic  inch  of  water  weighs  .0361  pound  and  the  height 

f\C\  C  O  C 

of  water  column  to  balance  the  chimney  effect  will  be  -7:^7-  =  .70". 

.UoU-L 

In  other  words,  the  chimney  will  produce  .70"  draft  under  the 
above  conditions. 

When  a  supply  of  heated  air  or  gas  is  delivered  to  the  bottom 
of  the  chimney,  a  flow  will  be  maintained  according  to  the 
familiar  formula  for  falling  bodies: 


where  V  =  velocity  in  feet  per  second; 
G  =  acceleration  due  to  gravity; 
#  =  head  or  "draft." 

If  the  gases  passed  up  the  chimney  without  friction,  the 
velocity  w^ould  be  proportional  to  the  square  root  of  the  draft, 
but  on  account  of  friction  against  the  inside  surface  of  the 
chimney,  the  flow  is  somewhat  retarded.  The  actual  draft  at 
the  base  of  a  chimney  is,  therefore,  dependent  upon  the  gas 
velocity  and  the  character  of  the  inside  surfaces  as  well  as 
upon  the  height  and  the  temperature  difference  between  out- 
side air  and  chimney  gases.  The  difference  in  weight  and  the 
actual  draft  available  in  an  operating  chimney  represents  the 
chimney  friction  and  the  increases  with  the  velocity.  When 
a  chimney  is  overloaded,  the  available  draft  drops  rapidly  due 
to  excessive  friction  as  may  be  seen  from  the  curves  of  chimney 
capacity  (Fig.  56). 

The  capacity  of  a  chimney  is  determined  by  the  weight  of 
gas  it  will  handle  in  a  given  time  and  it  therefore  follows  that 
the  density  must  be  considered  as  well  as  the  velocity.  An 
increase  in  gas  temperature  will  increase  the  velocity  but  there 


132  MECHANICAL  STOKERS 

is  also  an  increase  in  friction  and  a  decrease  in  density.  Where 
the  gas  temperature  is  about  625°  F.  the  capacity  of  a  chimney 
has  reached  the  maximum,  and  a  further  increase  in  tempera- 
ture will  increase  the  draft  intensity  but  the  weight  of  gas 
handled  will  decrease  because  the  density  will  decrease  more 
rapidly  than  the  velocity  will  increase. 

The  following  formula  can  be  used  for  determining  the 
friction  loss  in  chimneys: 

fW2CH 
Formula  A  .  AD  =J    ^3     , 

in  which    D  =  draft  loss  in  inches  of  water; 

W  —  weight  of  gas  in  pounds  per  second; 
C  =  perimeter  of  chimney  in  feet; 
H  =  height  of  chimney  in  feet; 
/=a  constant  with  the  following  values  at  sea  level: 

.0015  for  steel  chimneys,  temperature  of  gases  600°  F. 

.0011  for  steel  chimneys,  temperature  of  gases  350°  F 

.0020  for  brick  lined  chimneys,  temperature  of  gases  600°  F. 
.0015  for  brick-lined  chimneys,  temperature  of  gases  350°  F. 

The  static  draft  will  be  KH  in  which  K  is  a  constant,  hav- 
ing the  following  values  based  on  60°  F.  outside  air  and  14.7 
Ibs.  per  sq.  in.  atmospheric  pressure. 

Temperature 

Chimney  Gases  Constant  K 

750  .0084 

700  .0081 

650  .0078 

600  .0075 

550  .0071 

500  .0067 

450  .0063 

450  .0058 

350  .0053 

The  available  draft  will  be 

(fW*CH\ 
-- 


The  curves  (Figs.  55  and  56)  have  been  plotted  from 
results  calculated  by  this  method  and  will  be  found  suffi- 
ciently accurate  for  all  practical  purposes.  It  should  be  noted 


DRAFT 


133 


that  the  formula  for  chimney  performance  are  based  on  carry- 
ing capacity  in  terms  of  weight  and  in  determining  the  cor- 
responding horse  power,  a  value  must  be  established  for  the 
weight  of  gas  handled  per  horse  power.  Sixty  pounds  of  gas 
per  horse  power  is  an  average  value  for  good  operation  and 
where  .there  is  not  a  large  amount  of  leakage  through  boilers 
not  in  service.  Where  a  number  of  boilers,  some  of  which  are 
idle,  are  served  by  one  chimney,  this  leakage  will  be  large  and 
it  should  be  taken  into  account  in  determining  the  weight  of 
gas  handled  per  horse  power  developed. 

Draft  at  G>ase  of  Stack-  Inches  Water 
0.1      02     OA     0.4      0.5      0.6      0.1      0.8     0.9       1.0      I.I 


1000 
0 


Escaping 
Gasjemp 


>TSl  For  Stack  Heights  other  than 
100 Ft.  -  Multiply  tne  Curves  Value 
•by  the  Rrfio{  of  Height  to  100  ~i 


120,000  180,000         240,000      300,000 

Lbs.  of  Flue  Gas  per  Hr. 


0  90,000 

FIG.  55.  —  Performance  of  100'  Stacks  from  36"  to  90"  Diameter. 


As  an  illustration  of  the  use  of  these  curves,  assume  that  it 
is  desired  to  determine  the  correct  chimney  size  for  2,000  boiler 
H.P.  output.  It  is  first  necessary  to  establish  values  for 
weight  of  gas  per  H.P.  and  exit  temperature.  Sixty  Ibs.  of 
gas  per  H.P.  represents  good  operation  and  from  seventy  to 
seventy-five  Ibs.  if  fair.  First,  locate  the  intersection  of  the 
horizontal  line  for  2,000  H.P.  with  the  diagonal  line  represent- 
ing weight  of  gas  per  H.P.,  then  project  vertically  to  the 
curved  lines  of  chimney  diameter.  A  horizontal  line  through 
this  intersection  will  in  turn  intersect  the  diagonal  lines  repre- 
senting escaping  gas  temperature.  A  vertical  line  is  then 
drawn  through  this  latter  intersection  to  the  scale  of  chimney 


134 


MECHANICAL  STOKERS 


draft  at  the  top  of  the  diagram  and  the  draft  that  would  be 
produced  by  a  chimney  of  the  given  diameter  and  100  ft.  high 
is  read  from  this  scale.  For  example,  a  60"  chimney  100" 
high  would  produce  a  draft  of  only  .39"  with  a  load  of  2,000 
H.P.  In  general  it  may  be  said  that  a  chimney  that  will  not 
produce  at  least  .55"  draft  with  gases  at  500°  F.,  for  each 
100  ft.,  in  height,  is  overloaded  and  the  frictional  resistance  is 
too  great  due  to  excessive  velocities.  The  60"  chimney  would 
therefore  be  too  small.  The  selection  would  lie  between  the 

Draff  df  Base  of  Stack-  Inches  of  Water 
0  .10 .15      .25      35     .45      .55     .65      .15      .85     .95     1.05     US     t.25> 


eooo 


HOTE:  For  Stack  Heights  of  her  than 
'  100  Ft- Multiply  the  Curves  Value 
by  the  Ratio  ofHewht  to  100 


§  I  I  §  §  I  ill  1 

§•  s-  S  s    <£  s"  s   sf  g   x 

ucu^^m 


Lbs.of  Flue  Gas  per  Hr 
FIG.  56. — Performance  of  100'  Stacks  from  96"  to  144"  Diameter. 


72"  size  which  will  produce  .59"  draft  per  100  ft.  and  the 
78"  which  will  produce  .62"  per  100  ft.  It  is  apparent  that 
for  a  given  draft  at  the  base  of  the  stack,  a  lesser  height  will 
be  required  with  the  greater  diameter. 

Assume  that  1.10"  must  be  available  at  the  base  of  the 


stack,  the  72"  size  must  be         X  100  =187  ft.  high,  while  the 


78"  size  must  be        X  100  =177  ft.  high. 

\)Z 

The  choice  between  these  two  sizes  will  depend  upon  first 
cost  and  such  local  conditions  as  may  effect  the  selection. 


DRAFT  135 

In  the  case  of  natural  draft,  the  static  draft  must  overcome 
all  resistances  to  flow  which  are  as  follows : 

1.  Loss  through  fuel  bed  and  grate 

2.  Loss  through  boiler 

3.  Pressure  required  to  create  velocity  of  gases  leaving  the 

boiler 

4.  Damper  loss 

5.  Breeching  loss 

6.  Friction  loss  in  chimney 

A  number  of  formulae  have  been  developed  for  determining 
certain  of  these  amounts  but  they  depend  to  such  an  extent 
upon  the  values  selected  for  certain  constants  that  their  ap- 
plication is  of  doubtful  value.  It  is  known  that  the  temperature 
of  gases  in  a  chimney  is  less  near  the  top  than  at  the  point 
where  the  gases  enter,  due  to  radiation  and  to  leakage  of  cold 
air  into  the  chimney,  but  no  data  are  available  from  which 
the  amount  of  this  difference  can  be  determined  accurately. 
The  relation  between  static  draft  and  that  available  for  any 
given  operating  condition  will  depend  upon  a  number  of 
variables,  the  principal  ones  being  the  diameter  of  the  chimney, 
the  nature  of  its  surface,  and  the  velocity  of  the  gases. 

Of  the  above  losses,  the  first  and  second  are  most  important 
and  of  greatest  amount.  The  method  of  analyzing  these  losses 
can  best  be  explained  by  reference  to  Pig.  57,  which  shows 
a  cross  section  through  boiler  and  stoker.  Provision  should 
be  made  to  take  draft  readings  at  the  points  indicated  and 
connections  to  the  draft  gauges  should  be  carefully  made  to 
insure  an  absence  of  leaks  which  affect  the  accuracy  of  the 
readings.  If  these  readings  are  to  be  of  any  value,  they  must 
be  taken  under  operating  conditions,  with  a  good  fire  on  the 
grates  and  gas  analyses  should  be  made  to  determine  the  com- 
bustion conditions.  Notes  should  be  made  recording  the  fuel 
bed  conditions,  gas  analyses  and  ratings  developed.  These  data 
arc  necessary  as  they  affect  the  draft  losses  and  no  conclusions 
can  be  drawn  from  draft  readings  unless  accompanied  by  such 
information.  In  case  an  unusual  loss  is  encountered  between 
any  two  points,  such  as  B  and  C,  the  draft  gauge  can  be 
corrected  as  shown  to  read  this  loss  directly  and  the  effect  of 


138 


MECHANICAL  STOKERS 


any  change  in  conditions  will  be  detected  immediately.  The 
values  given  represent  actual  results  of  an  investigation  but 
are  used  merely  for  purposes  of  illustration  and  not  to  establish 
standard  values  for  the  type  of  boiler  shown. 

The  performance  of  a  chimney  was  first  presented  in  con- 
venient form  by  William  Kent  in  1884  and  the  table  of  chimney 
sizes  calculated  from  the  formula  given  has  been  almost  uni- 
versally used  by  engineers  since  that  time.  This  table  is  based 


4th.  Pass  8.24° ' 
Ratio  1  to  &38 


2nd.  Pass  14.13° 
Ratio  1  to  4.82 


Total  .46" 


NOTES:- 

fuel  Bed  about  6Mthick    „ 
COZ-I2.4%,   02-5.97o,C.O-0% 
RatinafrornFlow 
Meter  Readings  145% 
Ignition  Good 
No  Holes  in  Fire 
eideWalls'underArch 
Will  Bum  the  Bare  Hand 


FIG.  57. — Analyzing  Draft  Losses  through  Boilers. 

on  the  use  of  five  pounds  of  coal  per  boiler  H.P.,  and  twenty 
pounds  of  air  per  pound  of  coal.  The  height  of  the  chimney 
must  be  determined  independently  of  this  table  by  the  draft 
which  must  be  available  at  the  base  of  the  chimney  to  overcome 
the  various  losses  of  fuel  bed,  boiler,  damper  and  breeching  at 
the  ratings  for  which  the  installation  is  designed  and  many 
mistakes  have  been  made  by  engineers  due  to  their  failure  to 
make  proper  allowance  for  these  losses. 


DRAFT  137 

A  chimney  which  would  be  of  the  proper  size  to  produce 
draft  for  two  five  hundred  horsepower  boilers  operating  at 
rating,  would  be  entirely  unsuitable  for  one  of  these  boilers 
operating  at  200%  rating  for  the  reason  that  the  draft  loss 
through  the  fuel  bed  and  the  boiler  is  very  much  greater  at 
200%  rating  than  at  rating  and  the  chimney  which  was  right 
for  two  500  H.P.  boilers  at  rating  would  not  produce  the  draft 
intensity  required  to  overcome  these  losses. 

In  the  ordinary  chimney  calculations,  sea  level  conditions 
of  atmosphere  are  assumed.  The  error  due  to  this  assumption 
is  slight  for  elevations  up  to  1,000'  and  it  is  not  customary  to 
make  corrections  but  when  plants  are  located  at  greater  alti- 
tudes, it  is  necessary  to  make  allowance  for  the  existing  at- 
mospheric conditions.  Owing  to  the  lower  atmospheric  pres- 
sure at  high  altitudes,  the  draft  intensity  produced  by  a  chim- 
ney under  a  given  set  of  conditions  is  less  than  at  sea  level 
and  an  increase  in  height  is  necessary.  The  density  of  the 
air  and  flue  gases  being  less  at  the  higher  altitudes,  an  increase 
in  cross  sectional  area  is  necessary  to  handle  the  same  weight 
of  gas  at  a  given  velocity.  Due  to  the  lower  density,  there 
is  slightly  less  friction  for  a  given  velocity  but  this  is  so  small 
that  it  can  be  disregarded. 

In  proportioning  a  chimney  for  high  altitudes,  one  is  first 
selected  which  would  be  used  under  sea  level  conditions  and 
the  height  and  diameter  are  then  increased  by  the  amount 
required  to  correct  for  the  given  altitude  as  shown  in  Fig.  58. 

Since  the  height  of  the  chimney  depends  upon  the  draft 
which  will  be  required  at  its  base,  it  is  obviously  very  im- 
portant that  the  various  losses  which  must  be  overcome  be 
carefully  considered  and  accurately  estimated.  Failure  to  do 
this,  may  result  in  a  serious  mistake  and  one  which  is  very 
difficult  to  correct. 

The  various  losses  outlined  above  will  be  considered 
separately. 

First,  "Loss  through  fuel  bed  and  grate."  In  the  de- 
sign of  a  stoker  installation,  one  of  the  first  things  which 
must  be  determined  is  the  combustion  rate  as  outlined  in 
Chapter  VIII.  This  combustion  rate  being  known,  the  fuel 
bed  resistance  can  be  approximately  determined  for  natural 


138 


MECHANICAL  STOKERS 


draft  stokers  from  Fig.  59  and  for  forced  draft  underfeed 
stokers  from  Fig.  60. 


1000 


2000       3000       4000        5000       6000 
Altitude,  Feet  Above  Sea  level 


OOO 


8000       9000      10000 


FIG.  58. — Altitude  Corrections  for  Chimney  Dimensions. 

It  should  be  understood  that  these  curves   are   only  ap- 
proximate and  that  actual  performances  may  differ  consider- 


\ 

PerC 

enl-  Solids 

1  AH 

r;c.+Ash)/ 

V 

V 

M 

^D 

^ 

\ 

A  -40 
B-50 

/, 

/ 

/ 

X 

x^ 

\ 

"C-60 
D-70 

Z 

'/• 

/\ 

x 

\ 

rE- 

»U 

i 

// 

/, 

/ 

\ 

/ 

/\ 

/ 

\ 

/// 

y\ 

3 

\ 

/ 

///, 

/ 

i 

\ 

I 

Y 

1 

\ 

i 

\ 

j 

50    45     40     35     30     25     20     15 
Coal  per  Sq.ft. per  Hr.- Pounds 


10      5      0 


.10  '.20  .30    .40    .50    .60    .10    .80   .90    1.0 
Draft-Inches  of  Water 


FIG.  59. — Furnace  Draft  Required  for  Natural  Draft  Stokers. 

ably  from  the  values  given.  The  reason  for  this  is  that  the 
curves  fail  to  take  into  account  two  of  the  principal  factors 
affecting  fuel  bed  resistance,  namely  the  dust  and  moisture 


DRAFT 


139 


contents  of  the  fuel.     Commercial  screenings  will  vary  in  size 
approximately  as  follows: 


Overl 
Maximum  .......     50% 

Minimum  .......       2% 


Over£"  Over  J"  Over  |"  Through  \" 

25%         15%          7%  3% 

20%        25%         15%  38% 


Hundreds  of  draft  readings  have  been  taken  on  tests  where 
the  combustion  rates  were  being  carefully  recorded  but  no 
information  has  been  secured  as  to  the  percentage  of  dust 
in  the  fuel.  The  relation  of  draft  to  combustion  rate  is  affected 
to  such  an  extent  by  the  percentage  of  fine  dust  in  coal  that 
without  this  information  the  draft  readings  and  combustion 


\ 


Per  Cent  Solids(F.C.+ Ash) 
-A -40 


9t 


;A   .-B   -C 


100   90    80    70    60     50    40    30    20 
Coal  per  Sq. Ft.  per  Hr -Pounds 


10     0 


12345678 
Wind  Box  Pressure -Inches  of  Water 


FIG.  60. — Windbox  Pressure  Required  for  Underfeed  Stokers. 

rates  are  of  little  value.  A  fuel  containing  40%  of  dust  that 
will  pass  through  a  %"  round  screen  can  be  burned  at  only 
about  60%  of  the  rate  which  can  be  secured  with  the  same 
draft  from  coal  containing  only  5  or  10%  of  dust.  It  is  for 
this  reason  that  such  wide  discrepancies  exist  in  the  drafts 
required  to  burn  fuel. 

It  has  been  found  that  by  adding  a  sufficient  amount  of 
moisture  to  agglomerate  this  dust,  the  fuel  bed  resistance  can 
be  materially  re'duced  and  the  combustion  rate  with  the  given 
amount  of  draft  correspondingly  increased.  This  explains  the 
improved  combustion  which  often  results  from  adding  moisture 
to  fuel  and  which  is  often  ascribed  to  chemical  action  of  the 
moisture.  The  action  is  entirely  mechanical  and  is  due  to 
reduced  fuel  bed  resistance. 


140 


MECHANICAL  STOKERS 


The  percentage  of  solids  (F.  C.  and  ash)  in  coal  also  affect 
the  draft  required  for  combustion.  A  coal  high  in  volatile 
can  be  burned  at  a  given  rate  with  less  draft  than  a  low 
volatile  coal,  not  because  less  air  is  required  but  because  it 
is  the  solid  matter  and  not  the  volatile  that  determines  the 
character  of  the  fuel  bed  and  the  amount  of  air  that  can  be 
drawn  in  with  a  given  draft.  The  curves  (Fig.  59)  show  to 
what  extent  this  factor  influences  the  draft  required.  As  an 
example,  a  thirty-pound  combustion  rate  with  coal  having 
F.  C.  w+.  ash  =  50%  will  require  .21"  furnace  draft  while  a 


1.10 
1.00 
0.90 
0.80 
O.TO 
0.60f 
0.50 
040 
tt30 
020 
0.10 
0 


A  4   Pass  Vertical  Baffle    14 Tubes  High 

_B  3      »  »  »          16     »>         w« 

C  3      »  '»  >'          14     »» 

D  3 

•  E  3 

-I  ! 


»    Sterling  or  Vertical 


Heine  Type 


100 


140 


160  180          200         220 

Per  Cent  Boiler  Rating 


260 


280        300 


FIG.  61. — Draft  Losses  through  Boilers. 

coal  having  F.  C.  +  ash  =  70%  will  require  .38"  draft  for 
the  same  combustion  rate. 

Second:  "Loss  through  boiler."  The  draft  loss  through 
boilers  varies  through  wide  limits.  With  the  great  number 
of  boilers  of  different  designs  and  the  variety  of  baffle  arrange- 
ments, installed  to  meet  particular  local  conditions,  it  is  impos- 
sible to  establish  values  for  draft  losses  which  can  be  of 
general  application  and  it  is  advisable  to  secure  from,  the 
manufacturer,  figures  for  the  particular  boiler  and  baffle  ar- 
rangements which  may  be  under  consideration. 

The  curves  shown  on  Fig.  61  may  be  used,  however,  for 
preliminary  calculations. 


DRAFT  141 

Third:  "Pressure  required  to  create  velocity  of  gases  leaving 
the  boiler. ' '  The  draft  required  to  create  velocity  of  the  gases 
leaving  the  boiler  is  not  an  important  item.  At  a  velocity 
of  the  20'  per  second,  the  draft  required  is  about  .05"  at  a 
temperature  of  550°  and  for  30'  per  second,  the  draft  required 
is  about  .11".  There  is,  however,  a  certain  chimney  effect  in 
the  boiler  itself  due  to  the  vertical  height  from  the  fuel  bed 
to  the  damper  which  is  usually  sufficient  to  offset  the  loss 
required  to  create  the  velocity.  This  is  clearly  shown  in  the 
case  of  some  large  boilers  operating  on  forced  draft  running 
with  practically  atmospheric  pressure  in  the  furnace  which  can 
be  operated  at  rating  with  atmospheric  pressure  or  a  slight 
pressure  above  atmosphere  just  below  the  boiler  damper.  In 
this  case,  the  chimney  effect  of  the  boiler  is  sufficient  not  only 
to  create  the  velocity,  which  is  low,  but  also  to  overcome  the 
boiler  draft  loss, 

Fourth:  "Restriction  at  damper."  There  should  be  no  re- 
striction at  the  boiler  damper  if  it  were  wide  open  but  the 
areas  through  boiler  dampers  are  often  inadequate  especially 
when  the  boilers  are  operated  at  high  ratings.  As  dampers 
are  often  installed  there  is  a  sudden  change  of  direction  either 
when  the  gases  enter  the  damper  opening  or  just  as  they  leave 
which  is  responsible  for  a  certain  amount  of  draft  loss.  When 
boilers  are  operated  at  high  ratings,  the  gas  velocity  through 
the  average  damper  will  exceed  30'  per  second  and  there  will 
be  a  draft  loss  of  about  .1".  In  many  cases,  however,  dampers 
are  installed  in  such  a  manner  that  the  loss  is  two  or  three 
times  this  amount,  due  to  the  fact  that  the  path  of  the  gases 
through  the  damper  opening  is  such  that  when  the  damper  is 
wide  open  it  interferes  with  the  free  flow  of  gases  through  the 
opening  on  both  sides  of  the  damper  itself.  The  location  of 
the  damper  and  the  path  of  the  gases  both  entering  and  leaving 
should  be  carefully  considered  to  insure  a  free  passage  if 
unnecessary  draft  losses  at  this  point  are  to  be  avoided. 

Fifth:  "Breeching  losses."  Accurate  data  upon  which  draft 
losses  through  a  given  breeching  may  be  determined  are  not 
available.  The  effect  of  changes  in  cross  section  of  the  breech- 
ing, disturbances  in  the  flow  caused  by  successive  boilers  dis- 
charging into  the  breeching,  the  shape  of  the  gas  passage  and 


142  MECHANICAL  STOKERS 

the  nature  of  its  surface  cannot  be  determined  with  a  degree 
of  accuracy  which  permits  of  general  application.  The  best 
posible  guide  in  the  design  of  a  breeching  is  an  accurate  record 
of  draft  losses  in  a  similar  breeching  where  gas  velocities  are 
approximately  the  same  as  those  for  which  the  breeching  is 
being  designed. 

There  are  a  number  of  " thumb  rules"  for  proportioning 
breeches  in  accordance  with  boiler  H.P.,  grate  area,  or  some 
other  known  unit,  but  they  all  neglect  one  or  more  factors 
which  have  an  important  bearing  on  the  correct  design.  It 
is  necessary,  therefore,  to  analyze  the  conditions  in  sufficient 
detail  to  determine  the  gas  volumes  to  be  handled  and  the 
breeching  areas  proportioned  to  give  the  desired  velocity. 

Formula  A  (page  132)  for  friction  losses  in  chimneys  is  ap- 
plicable to  breechings  but  is  less  accurate  due  to  the  disturbances 
created  by  successive  boilers  discharging  into  the  breeching  with 
consequent  sudden  changes  in  the  direction  and  velocity  of  flow. 

In  general,  it  may  be  said  that  there  will  be  a  loss  of  .1" 
for  every  50'  of  straight  breeching  for  breechings  having  not 
less  than  25  sq.  ft.  cross  sectional  area  and  a  velocity  not 
exceeding  30'  per  second.  The  draft  loss  in  a  right  angled 
bend  will  also  be  about  .1"  for  a  velocity  not  exceeding  30'  per 
second.  If  the  area  of  the  breeching  is  small,  the  draft  loss 
is  increased  and  in  such  cases,  it  is  usually  preferable  that  the 
velocity  be  reduced  rather  than  that  the  chimney  be  made 
high  enough  to  overcome  the  additional  friction.  A  good  rule 
to  follow  in  this  connection  is  that  the  velocity  in  feet  per 
second  should  not  be  greater  than  the  area  of  the  breeching 
in  square  feet.  This  of  course  becomes  absurd  for  very  small 
breechings  but  can  be  followed  for  areas  as  small  as  twelve 
to  fifteen  square  feet.  For  larger  breechings,  a  velocity  of 
from  twenty-five  to  thirty  feet  per  second  will  be  found  satis- 
factory and  this  can  be  increased  to  thirty-five  feet  for  short 
distances. 

A  circular  breeching  offers  less  resistance  to  the  flow  of  gas 
than  one  of  square  or  rectangular  section.  A  breeching  of  a 
given  diameter  has  about  the  same  draft  loss  for  a  given 
condition  as  a  square  breeching  whose  sides  equal  the  diameter 
of  the  round  breeching.  In  the  case  of  rectangular  breechings, 


DRAFT 


143 


the  ratio  of  surface  to  cross  sectional  area  is  greater  than  for 
either  the  round  or  square  breeching  and  the  carrying  capacity 
for  a  given  draft  loss  becomes  less.  By  the  use  of  curve  (Fig. 
62)  a  rectangular  breeching  may  be  directly  compared  to  its 
equivalent  circular  or  square  section  of  equal  carrying  capacity. 
Sharp  turns  or  sudden  changes  in  cross  section  are  to  be 
avoided  and  a  construction  should  be  employed  that  is  air- 
tight when  new  and  can  easily  be  kept  tight.  Suitable  doors 
should  be  provided  for  inspection  and  cleaning  and  where 


40  50  60 

Diameter  of  Circle 


100 


FIG.  62. — Circular  Equivalents  of  Rectangles  for  Equal  Friction  per  Unit  of 

Length, 

radiation  will  cause  excessive  temperatures  in  places  where 
repair  men  may  be  required  to  work,  the  breeching  should  be 
insulated.  Underground  flues  are  almost  invariably  unsatis- 
factory and  should  be  avoided  wherever  possible. 

It  is  sometimes  difficult  to  secure  a  simple  straight  breech- 
ing on  account  of  local  conditions  but  if  the  importance  of 
such  construction  is  fully  appreciated,  the  necessary  changes 
can  usually  be  made  to  permit  of  a  suitable  layout.  Construc- 
tion difficulties  are  soon  forgotten  after  operation  has  begun 
but  the  operating  engineer  who  has  to  contend  with  a  poor 
breeching,  works  at  a  disadvantage  as  long  as  the  plant 


144 


MECHANICAL  STOKERS 


operates.  In  one  extreme  case  of  bad  design,  two  90°  turns 
and  twenty  feet  of  length  were  added  to  avoid  moving  a  16" 
steam  line  which  could  be  cut  out  over  the  week  end  and 
changed  without  affecting  the  plant  operation. 

Fig.  63  shows  the  method  of  analyzing  draft  conditions 
in  a  breeching  and  incidentally  brings  out  some  defects  in  the 
design  shown  although  the  areas  were  ample  and  the  gas 
velocity  at  no  point  greater  than  twenty-five  feet  per  second. 

The  dampers  when  open  project  into  the  breeching  and 
seriously  interfere  with  the  gas  flow  both  by  restricting  the 


Boiler  No.  1    Boiler  No.2      Boiler  No.3   BoilerNo.4  BoilerNo.5    BoilerNo.& 

NOTE'.  Letters  Show  Location  of  Draft  Readings 
A-.44"    B-.62"  C-.G8"   D-.85"  E-.5S"    F-.SO"    G'.45"  H-.921' 

FIG.  63. — Analyzing  Draft  Losses  in  Breechings. 

breeching  area  and  by  improperly  directing  the  gas.  At  A, 
the  gas  passing  through  the  left  side  of  the  damper  opening 
is  directed  against  the  top  of  the  breeching  and  must  make 
a  sudden  change  in  direction  resulting  in  an  unnecessary  draft 
loss.  At  By  the  same  condition  exists  and  in  addition,  the 
net  breeching  area  is  reduced  to  about  two-thirds  of  the  full 
area,  resulting  in  a  gas  velocity  at  this  point  considerably  greater 
than  that  for  which  the  breeching  was  designed.  This  condition 
exists  at  all  boiler  dampers  and  is  a  serious  defect.  Where  the 
two  branches  meet  and  go  to  the  chimney,  there  is  a  sudden 


DRAFT  145 

change  in  direction  and  a  serious  disturbance  to  flow  by  the 
manner  in  which  the  two  streams  of  gas  come  together.  This 
is  shown  by  the  large  draft  loss  between  points  C  and  H.  The 
sudden  reduction  in  width  between  point  H  and  the  entrance 
to  the  chimney  is  also  responsible  for  an  unnecessary  loss  which 
could  easily  have  been  avoided. 

This  breeching  would  be  a  good  design  instead  of  a  very 
poor  one  if  the  following  changes  had  been  made:  First,  raise 
the  breeching  enough  to  bring  the  top  of  the  boiler  dampers 
when  wide  open  down  to  the  bottom  of  the  breeching,  thereby 
allowing  the  full  area  for  gas  passage;  second,  eliminate  the 
right  angle  turn  where  the  two  branches  meet  the  connection 
to  the  chimney;  third,  reduce  the  width  of  this  chimney  con- 
nection gradually  instead  of  suddenly.  The  changes  could 
have  been  made  with  only  a  slight  increase  in  cost  and  no 
structural  difficulties  would  have  been  encountered  in  this  par- 
ticular case. 

The  sum  of  losses  one  to  five  inclusive,  is  the  actual  draft 
which  must  be  available  at  the  base  of  the  chimney  and  when 
determining  the  chimney  height  necessary  to  produce  this  draft, 
the  maximum  temperature  of  the  atmosphere  in  the  given 
locality  must  be  used  because  the  draft  depends  not  upon  the 
temperature  of  the  gases  in  the  chimney  but  upon  the  difference 
between  this  temperature  and  that  of  the  outside  air. 

There  has  been  much  discussion  of  the  merits  of  individual 
chimneys  as  compared  with  one  large  chimney  for  several 
boilers.  The  individual  unit  has  the  advantage  of  being  pro- 
portioned for  only  one  boiler  and  no  allowance  must  be  made 
for  leakage  through  dampers  of  idle  boilers.  Repairs  can  be 
made  when  necessary  without  interfering  with  plant  operation, 
there  is  no  breeching  loss  to  consider  and  close  fitting  boiler 
dampers  are  not  necessary.  Where  boiler  units  are  small,  how- 
ever, the  cost  is  excessive  and  this  consideration  will  result 
in  the  selection  of  one  chimney  for  a  number  of  units.  A 
similar  selection  is  sometimes  made  for  the  sake  of  appearance. 
In  general  it  may  be  said  that  while  the  chimney  serving  a 
group  of  boilers  must  be  made  higher  to  take  care  of  breeching 
losses  and  damper  leakage  there  are  no  controlling  factors 
except  cost  and  appearance. 


146  MECHANICAL  STOKERS 

The  efficiency  of  a  chimney  will  depend  upon  the  basis  on 
which  the  calculations  are  made.  Since  the  flow  of  gases  is 
maintained  due  to  temperature  difference  inside  and  outside 
the  chimney,  the  efficiency  is  measured  by  the  amount  of  heat 
expended  for  this  purpose.  The  foot  pounds  of  energy  required 
to  handle  a  given  weight  of  gas  represents  a  very  small  per- 
centage of  the  energy  delivered  to  the  chimney  in  the  form 
of  heat  and  on  this  basis,  the  efficiency  would  not  exceed  .06% 
under  average  conditions.  While  the  actual  efficiency  is  very 
low,  the  chimney  has  many  advantages  as  a  draft  producer. 
It  has  no  moving  parts  and  does  not  require  daily  attention. 
The  cost  of  upkeep  is  low  and  the  possibility  of  failure  is 
remote.  The  heat  necessary  to  produce  the  draft  has  been 
rejected  by  the  boilers  and  might  be  considered  a  waste 
product.  With  these  advantages,  the  chimney  would  appear 
to  be  ideal  for  producing  draft  but  the  fact  that  other  methods 
are  often  employed,  shows  that  there  are  conditions  which  the 
chimney  cannot  meet. 

Induced  Draft. — With  the  draft  requirements  in  many 
plants  and  the  temperatures  at  which  the  gases  leave  the  boiler 
heating  surface,  a  chimney  of  less  than  200'  in  height  is  suffi- 
cient. Many  of  the  smaller  installations  require  chimneys  of 
not  over  100'  in  height  and  except  in  unusual  cases,  chimneys 
of  over  250'  are  not  built. 

The  use  of  an  economizer  has  two  very  important  effects 
on  the  draft  producing  apparatus :  first,  it  lowers  the  tempera- 
ture of  the  gases,  and  since  the  ability  of  the  chimney  to 
produce  draft  depends  upon  the  difference  of  temperature  be- 
tween the  air  and  gas,  it  reduces  the  effectiveness  of  the  chim- 
ney, requiring  a  much  greater  height  to  produce  a  given  draft 
intensity;  second,  the  economizer  offers  a  certain  amount  of 
resistance  to  the  flow  of  gases  which  must  be  overcome  by 
the  chimney  draft.  A  greater  draft  intensity  at  the  base  of 
the  stack  is  therefore  required  when  the  economizer  is  used. 
Considering  the  decreased  temperature  and  the  increased  gas 
resistance,  it  would  often  require  a  chimney  400  or  500'  in 
height  to  produce  the  necessary  draft.  This,  of  course,  is  im- 
practical on  account  of  the  great  cost  and  some  other  means 
for  producing  draft  must  be  employed. 


I47J 

A  number  of  years  ago  many  installations  were  made  in 
which  one  economizer  served  all  the  boilers  in  the  plant  and 
delivered  the  gases  to  a  chimney.  The  installation  of  economiz- 
ers under  such  circumstances  often  resulted  in  an  actual  loss 
in  economy  instead  of  in  a  substantial  gain  which  should  have 
been  realized,  and  in  addition,  decreased  the  capacity  on  ac- 
count of  the  large  reduction  in  draft  available  for  burning 
fuel. 

The  induced  draft  fan  which  takes  the  gases  from  the 
breeching  at  a  pressure  below  the  atmospheric  pressure  and 
discharges  them  to  the  atmosphere  is  commonly  employed 
where  the  chimney  is  not  able  to  meet  the  conditions.  With 
this  arrangement,  any  draft  intensity  required  in  even  the  most 
unusual  cases  is  easily  secured.  Induced  draft  is  also  employed 
in  many  plants  where  a  high  chimney  would  detract  from  the 
appearance  of  the  building  or  its  surroundings. 

The  increasing  tendency  to  operate  boilers  at  higher  ratings 
introduces  two  conditions  tending  to  make  induced  draft  de- 
sirable if  not  absolutely  necessary:  first,  the  temperature  of 
escaping  gases  increases  with  the  rating  and  it  is  found  desir- 
able to  install  economizers  to  reclaim  the  large  amount  of  heat 
that  would  otherwise  be  lost;  second,  the  draft  loss  through 
a  boiler  increases  rapidly  with  the  rating  and  at  ratings  now 
developed  in  many  plants,  chimneys  of  commercial  sizes  do  not 
produce  sufficient  draft  to  overcome  these  high  losses. 

Induced  draft  fans  to  fulfill  any  set  of  conditions  met  with 
in  practice  can  be  selected  from  standard  designs  and  it  is 
therefore  not  necessary  in  the  selection  of  induced  draft  equip- 
ment to  go  further  than  to  determine  the  service  which  the 
fan  must  perform.  The  procedure  so  far  as  the  suction  at  the 
fan  inlet  is  concerned  is  practically  the  same  as  in  the  case 
of  chimney  design.  To  determine  the  volume  of  gas  to  be 
handled,  it  is  necessary  to  know  the  maximum  horse  power 
which  the  boilers  served  by  the  fans  will  deliver,  the  heating 
value  of  the  coal,  the  percentage  of  excess  air  in  the  gases 
entering  the  fan  and  the  temperature.  The  percentage  of 
excess  air  must  be  determined  from  a  knowledge  of  operating 
conditions  and  the  gas  temperatures  can  be  stated  with  a  fair 
degree  of  accuracy  by  the  builders  of  the  boilers  and  economiz- 


148 


MECHANICAL  STOKERS 


may  be  used  to  de- 
handled   per  unit   of 


ers.  With  this  information,  Fig.  64 
termine  the  cubic  feet  of  gas  to  be 
time. 

Induced  draft  fans  are  of  two  general  types :  the  steel  plate 
and  multi-vane.  Steel  plate  fans  run  at  relatively  low  rotative 
speeds  and  are  usually  engine  driven,  the  engines  being  direct 
connected.  The  multi-vane  fans  run  at  higher  speeds  and  are 
suitable  for  turbine  or  motor  drive.  The  type  of  fan  selected 
will  depend  upon  local  conditions  and  individual  preference. 
Where  units  of  large  size  are  installed,  space  limitations  often 


& 


40 


10      II        12,      13       14       15       16       n       18       19      20      21       22      23     24      25 
Cubic  Feet  of  Air  per  Minute 

FIG.  64. — Volume  of  Air  per  Developed  Boiler  Horsepower. 


make  the  multi-vane  fan  the  only  one  which  can  be  conveniently 
installed. 

The  fan  drive  must  be  selected  with  a  clear  understanding 
of  the  nature  of  the  service  it  must  perform  and  the  drive 
should  be  of  sufficient  power  to  operate  the  fan  at  maximum 
capacity  under  the  most  unfavorable  conditions.  In  the  case 
of  a  steam-driven  unit,  the  engine  or  turbine  should  be  of 
sufficient  size  to  carry  the  full  load  with  an  abnormal  drop 
in  steam  pressure.  When  steam  is  low,  the  fan  must  handle 
the  maximum  amount  of  gas  if  normal  pressure  conditions  are 
to  be  re-established  and  if  the  fan  drive  is  not  of  sufficient 
power,  the  fan  will  slow  down  thereby  crippling  the  entire 
plant.  The  drive  should,  therefore,  be  designed  to  operate  on 


DRAFT  149 

a  pressure  somewhat  below  the  lowest  pressure  which  may 
ever  exist. 

The  exhaust  from  the  steam  unit  should  be  used  for  heating 
feed  water  or  for  other  useful  work  around  the  plant.  In  case 
exhaust  steam  is  not  required,  the  fan  should  be  motor  driven. 
In  the  case  of  turbine  drives,  the  fan  may  be  direct  connected 
to  a  low  speed  turbine  if  a  large  amount  of  exhaust  steam  can 
be  utilized  but  where  only  a  limited  amount  of  steam  is  desired, 
the  high  speed  geared  turbine  is  preferable  on  account  of  its 
lower  water  rate.  This  matter  should  be  carefully  studied  in 
connection  with  the  heat  balance  of  the  plant  and  in  no  case 
should  a  steam  driven  unit  be  installed  if  any  of  the  exhaust 
steam  is  to  be  wasted.  If  the  exhaust  is  properly  utilized, 
the  net  reduction  from  the  overall  efficiency  of  the  boiler  unit 
will  be  only  a  fraction  of  one  per  cent  but  if  the  exhaust  is 
to  be  discharged  to  the  atmosphere,  the  loss  may  be  as  high  as 
five  per  cent. 

A  chimney  which  is  properly  designed  for  a  given  plant 
condition  where  economizers  are  not  installed,  would  be  unable 
to  meet  the  requirements  if  economizers  were  added,  due  to 
the  fact  that  it  would  not  be  able  to  produce  sufficient  draft 
intensity  to  overcome  the  various  resistances.  In  the  case  of 
an  induced  draft  fan,  however,  it  usually  is  the  case  that  the 
same  fan  will  do  the  work  whether  economizers  are  installed 
or  not.  In  either  case  practically  the  same  weight  of  gas 
must  be  handled.  The  installation  of  economizers  merely  re- 
duces the  temperature  and  volume  and  increases  the  resistance 
to  be  overcome.  The  induced  draft  fan  operating  in  connection 
with  economizers  would  have  a  smaller  volume  to  handle  than 
would  be  the  case  without  economizers  and  due  to  this  decrease 
in  volume,  and  increase  in  density  the  fan  would  naturally 
produce  a  higher  suction.  The  one  just  about  offsets  the  other 
and  the  fan  is  therefore  able  to  meet  both  conditions,  requiring 
almost  exactly  the  same  horse  power  in  either  case. 

Forced  draft  is  required  for  those  stokers  carrying  heavy 
fuel  beds  and  designed  for  operation  at  high  rates  of  combus- 
tion. Pressures  under  the  grates  of  such  stokers  vary  from 
V  to  6"  and  the  increasing  use  of  such  stokers  has  brought 
about  the  development  of  a  number  of  fans  particularly  well 


150  MECHANICAL  STOKERS 

suited  to  this  class  of  work.  The  pressure  required  for  dif- 
ferent combustion  rates  on  stokers  using  forced  draft  is  subject 
to  the  same  correction  for  the  dust  content  of  the  fuel  that 
applies  to  natural  draft  stoker  work  and  (Fig.  60)  showing 
the  relation  of  wind  box  pressures  to  combustion  rates  are 
approximate  only  although  the  effect  of  the  dust  content  is 
less  marked  than  in  the  case  of  natural  draft  stokers. 

Where  forced  draft  is  employed  it  is  necessary  to  make 
calculations  of  the  amount  of  air  that  will  be  required.  It  is  pos- 
sible to  calculate  the  theoretical  amount  of  air  required  for 
combustion  as  outlined  in  Chapter  I  and  to  make  an  allowance 
for  excess  air  entering  the  combustion  process  and  for  leakage 
from  the  air  duct.  Fig.  64  shows  the  volume  of  air  in 
C.F.M.  per  boiler  H.P.  developed  for  various  efficiencies  and 
grades  of  coal.  For  example,  if  13,000  B.T.U.  coal  were  being 
burned  at  a  combined  efficiency  of  70%,  there  would  be 
required  12.8  cubic  feet  of  air  per  minute  per  H.P.  This 
allows  about  60%  excess  air  which  is  more  than  that  required 
for  good  operation  and  will  be  found  to  provide  ample  for  all 
reasonable  conditions. 

Forced  draft  fans  should  be  selected  of  ample  capacity  for 
the  maximum  demand  which  they  may  be  required  to  meet 
in  case  of  emergency.  The  data  available  on  forced  draft 
stoker  work  indicate  that  in  almost  every  installation,  the 
limiting  capacity  is  determined  by  the  ability  to  force  air 
through  the  fuel  bed.  In  the  case  of  natural  draft  stokers  an 
abnormally  high  draft  draws  such  quantities  of  air  through 
the  fuel  bed  near  the  zone  of  ignition  that  the  temperatures 
will  fall  and  ignition  become  sluggish,  thus  limiting  the 
capacity  which  can  be  secured.  Stokers  of  this  type  are  there- 
fore provided  with  arch  constructions  which  are  designed  with 
a  view  to  providing  sufficient  ignition  effect  for  the  ratings 
which  are  desired.  In  the  case  of  the  heavy  fuel  beds  carried 
on  forced  draft  stokers  there  is  probably  a  similar  limitation 
but  it  has  never  been  reached  in  practice.  The  limiting  factor 
is  either  the  ability  to  supply  air  or  to  discharge  the  refuse 
and  since  for  short  peak  loads  the  stokers  can  be  operated 
without  discharging  refuse,  the  ability  to  supply  air  is  the 


DRAFT 


151 


limiting  factor,  and  the  importance  of  ample  fan  capacity  is 
apparent. 

In  selecting  a  fan  for  high  altitudes,  the  density  of  the 
air  must  be  taken  into  account.  The  static  pressure  developed 
by  a  fan  at  any  given  speed  will  decrease  with  increased 
altitude  and  it  is  therefore  necessary  to  make  a  correction  as 
fan  performances  are  based  on  sea  level  conditions.  It  is  also 
necessary  to  correct  the  volume  of  air  to  give  a  weight  of  air 
equal  to  the  weight  at  sea  level.  It  will  be  found,  however, 
that  the  power  required  to  drive  the  fan  will  be  less  than  that 
calculated  for  sea  level  conditions  and  an  additional  correc- 
tion must  be  made  for  this  condition.  Fig.  65  can  be 

115 


85 


500 


2500 


1000  1500 

Altitude  in  Feet 

FIG.  65. — Altitude  Corrections  for  Fan  Performance. 


3000 


used  for  this  purpose.  It  will  be  seen  that  at  an  altitude  of 
3,000  ft.  the  volume  and  pressure  determined  for  sea  level  con- 
ditions must  be  increased  12%  and  that  the  power  will  be 
decreased  10%.  As  an  example,  assume  a  condition  requir- 
ing 25,000  C.F.M.  at  6"  static  pressure  at  sea  level.  At  an 
altitude  of  3,000',  the  volume  and  pressure  must  be  increased 
32%,  making  28,000  C.F.M.  against  a  static  pressure  of  6.72 
inches.  If  a  fan  were  selected  that  would  deliver  28,000  C.F.M. 
against  6.72  inches  at  sea-level  and  required  50  H.P.  for  its 
operation,  the  power  required  at  an  elevation  of  3,000  ft. 
would  be  (.9  X  50)  or  45  H.P. 

A  forced  draft  fan  to  be  suitable  for  stoker  work  must 
have  certain  characteristics.  Fans  are  designed  to  deliver  a 
given  volume  'of  air  against  a  given  static  pressure  when  run- 
ning at  a  specified  speed.  In  case  the  resistance  be  increased 


152  MECHANICAL  STOKERS 

and  the  speed  maintained  constant,  some  designs  will  deliver 
against  this  increased  resistance  only  a  small  percentage  of 
their  rated  volume.  Such  fans  are  evidently  not  suitable  for 
forced  draft  stoker  work.  In  case  of  careless  operation, 
abnormally  heavy  fires  may  be  built  up  increasing  the  pressure 
against  which  the  fan  must  operate  and  the  design  should  be 
such  that  the  fan  will  operate  against  this  increased  resistance 
with  only  a  slight  decrease  in  volume.  The  steel  plate  fan 
has  the  correct  characteristics  for  such  performance  and  mul- 
tivane  designs  have  also  been  developed  which  fulfill  this 
requirement  very  satisfactorily.  There  are  multivane  fans 
designed  for  different  service  which  do  not  have  this  charac- 
teristic and  they  should  not  be  used  for  forced  draft  service. 

The  selection  of  a  fan  drive  depends  largely  upon  local 
conditions.  Where  exhaust  steam  can  be  used,  steam  driven 
units  are  often  installed.  In  the  case  of  small  installations 
where  the  pressure  need  never  exceed  three  or  four  inches  of 
water,  the  slow  moving  steel  plate  fan  direct  driven  by  a 
steam  engine  is  often  used.  Larger  installations  usually  con- 
sist of  multivane  fans  turbine  driven  with  or  without  reduc- 
tion gears,  depending  upon  the  amount  of  exhaust  which  can 
be  used.  It  is  sometimes  desirable  to  install  some  steam  driven 
and  some  electrically  driven  fans  in  a  large  installation  in 
order  to  assist  in  maintaining  a  correct  station  heat  balance. 
In  plants  where  exhaust  steam  can  be  utilized,  the  net  charge 
against  the  forced  draft  equipment  represents  from  .25  to  .6 
of  1%  of  the  steam  generated.  Where  motor  driven  fans  are 
used,  the  net  deduction  will  be  from  .75  of  1%  to  ll/2%  but 
will  always  be  less  than  1%  where  efficient  generating  equip- 
ment is  installed. 

Boiler  units  of  800  H.P.  and  larger,  equipped  with  forced 
draft  stokers,  are  suitably  served  by  individual  fans.  This 
arrangement  has  the  advantage  of  short  connections  from  fan 
to  stoker  wind  box  and  simplifies  the  regulation  of  the  air 
supply  to  the  individual  units.  It  also  eliminates  the  resistance 
which  is  encountered  in  long  air  ducts  requiring  less  power  to 
operate  the  fans.  Smaller  units  are  usually  served  from  a 
common  air  duct  suplied  by  one  or  more  fans  conveniently 
located.  Owing  to  the  fact  that  air  is  delivered  to  the  stokers 


DRAFT  153 

at  atmospheric  temperature,  ducts  of  large  cross  section  are 
usually  not  required.  Air  velocities  of  from  30  to  50'  per 
second  are  usually  employed,  although  in  case  of  space  limita- 
tions considerably  higher  velocities  can  be  used  if  proper 
allowance  be  made  for  the  friction  in  the  duct.  For  steel 
ducts  there  will  be  a  pressure  drop  equal  to  the  pressure 
required  to  create  the  given  velocity  for  every  twenty  diameters 
of  duct  and  a  right  angled  turn  will  cause  a  drop  of  one  half 
this  amount.  For  construction  reasons,  ducts  are  usually  made 
square  or  rectangular  and  the  equivalent  round  duct  may  be 
readily  determined  from  Fig.  62. 

It  has  been  pointed  out  that  the  plant  heat  balance  affects 
the  selection  of  fan  drives.  Another  factor  which  must  be 
considered  is  the  nature  of  the  boiler  plant  load.  In  the  case 
of  an  electric  generating  station,  any  accident  which  affects 
the  electrical  system  seriously  will  throw  the  load  off  the 
boiler  house.  In  such  plants  motor  drives  can  be  safely 
employed  but  if  part  of  the  steam  goes  to  the  electric  generat- 
ing apparatus  and  a  considerable  amount  goes  into  process 
work,  a  failure  in  the  electrical  ends  of  the  plant  will  throw 
only  part  of  the  load  off  the  boilers.  In  such  cases,  it  is  desir- 
able either  to  install  some  steam  driven  fan  units  or  to  secure 
the  necessary  electric  power  from  an  independent  steam  driven 
house  generating  set. 


CHAPTER  VI 

FACTORS  AFFECTING  SELECTION  OF  STOKER 
EQUIPMENT 

When  stokers  are  applied  to  steam  generating  apparatus, 
the  individual  features  of  the  different  types  of  stokers  and 
their  influences  are  taken  into  account.  Especially  is  this 
necessary  for  proper  selection  of  stoker  equipment  to  attain 
certain  ultimate  results  of  the  complete  generating  unit  includ- 
ing the  stoker,  the  furnace  and  the  boiler. 

A  plan  to  investigate  the  stoker  for  a  special  combination 
requires  primarily  an  exact  definition  of  the  terms  used  in 
connection  with  the  units  making  up  the  complete  steam 
generating  apparatus.  For  example,  the  word  "boiler,"  is 
employed  indiscriminately  to  designate  either  the  boiler  proper 
or  the  combination  of  it  with  other  apparatus. 

The  brick  walls  which  enclose  the  boiler,  the  stoker  or  the 
furnace,  are  not  recognized  as  being  in  any  way  a  part  of  the 
metal  structure. 

The  stoker  is  that  portion  of  the  combination  which  pro- 
vides means  for  feeding  the  fuel  as  required,  supplying  the  air 
in  such  relative  proportions  as  to  cause  a  balancing  of  the 
quantities  of  air  and  combustible,  and  causing  the  air  and 
gases  to  mix  and  burn  without  smoke,  also  providing  means 
for  burning  the  coke  and  discharging  the  ashes. 

The  furnace  is  the  intermediate  element  located  between 
and  connecting  the  stoker  and  boiler,  wherein  the  process  of 
combustion  which  begins  at  the  grate  is  finished,  and  where  a 
sufficient  mixture  between  air  and  combustible  may  be  secured. 

In  selecting  stoker  equipment,  four  factors  are  considered. 
These,  in  the  order  of  their  importance,  are — 

1.  Load  to  be  carried. 

2.  Fuel  to  be  used. 

154 


SELECTION  OF  STOKER  EQUIPMENT  155 

3.  Draft  requirements. 

4.  Application  characteristics. 

LOAD   CONDITIONS 

The  character  of  the  load  to  be  carried,  that  is,  the  boiler 
horse  power  that  must  be  developed  and  the  time  element  in 
connection  with  increases  in  the  load,  is  of  prime  importance 
because  this  factor  is  generally  beyond  control;  that  is,  the 
characteristic  load  is  generally  established  by  the  particular 
industry.  It  is  the  function  of  the  stoker  and  boiler  unit  to 
carry  the  horse  power  requirements  through  the  many  varia- 
tions, at  a  minimum  cost,  and  above  all,  in  a  reliable  way. 

The  problem  of  a  manufacturer  who  has  a  steady  load 
and  a  comparatively  continuous  output  of  his  product,  is 
entirely  different,  insofar  as  stokers  are  concerned,  from  that 
of  the  Central  Station  operator  who  is  called  upon  to  meet 
almost  instantaneous  demands  for  steam  which  require  opera- 
tion of  boilers  at  from  100%  to  400%  of  boiler  rating. 

When  the  load  that  is  to  be  carried  is  determined,  a  study 
is  made  of  the  characteristic  curves  of  the  different  types  of 
stokers  showing  the  relation  between  the  load  and  the  com- 
bined efficiency  of  the  boiler  and  stoker.  Stoker  performance 
follows  quite  closely  a  certain  curve,  and  this  varies  accord- 
ing to  the  proportions  of  the  apparatus. 

Fig.  66  shows  the  relation  between  the  coal  burned  and 
the  combined  efficiency  of  boiler  and  grate  of  an  underfeed 
stoker.  It  has  a  wide  range  on  each  side  of  the  peak  of  the 
curve,  and  is  relatively  flat. 

A  range  in  combustion  rate  of  3  to  1,  and,  in  some  cases, 
4%  to  1,  is  possible  on  this  type  of  stoker  without  decreasing 
the  efficiency  very  much.  This  corresponds  roughly  from  70% 
to  200%  of  boiler  rating  for  some  sizes  of  underfeed  stokers, 
and  70%  to  300%  for  larger  sizes.  Fig.  67  shows  the  ability 
of  the  underfeed  type  of  stoker  to  handle  sudden  demands 
for  steam  without  very  much  preparation.  Without  forced 
draft,  sudden  changes  in  fuel-burning  rate  would  not  be  pos- 
sible. The  deep  fuel  bed  provides  a  reserved  capacity  so  that 
jumps  from  50%  to  200%  of  rating  can  be  made  without 
much  change  in  the  fuel-feeding  speed. 


156 


MECHANICAL  STOKERS 


SELECTION  OF  STOKER  EQUIPMENT 


157 


158 


MECHANICAL  STOKERS 


Fig.  68  shows  the  relation  of  load  to  combined  boiler  and 
stoker  efficiency  of  the  overfeed  stoker.  This  characteristic 
curve  has  about  the  same  shape  as  the  underfeed  but  does  not 
have  the  same  range.  The  most  efficient  point  for  this  condi- 
tion was  at  79%  of  boiler  rating,  and  the  maximum  horsepower 
was  225%  of  boiler  rating,  which  was  probably  the  maximum 
reserve  capacity  for  the  local  conditions  under  which  the  equip- 
ment was  designed. 

84 


LU    O 


Roney  Stoker  Red  Jacket  Coal  except  where  marked 


-lL 


n  is 


&£        10   80    90     100    110    120    130    140    150    160    110    180    190   200    210.  220 
Per  Cent  of  Rating  on  Basis  of  10  Sq.  Ft. of  Boiler  Heating  Surface =1  hp. 

FIG.    68. — Relation    between   Load  and  Combined   Efficiency  of  Boiler  and 
Stoker,  Front  Inclined  Overfeed  Stoker. 

Fig.  69  shows  the  relation  of  load  to  the  combined  boiler 
and  stoker  efficiency  for  different  fuel  bed  thickness  of  the 
chain  grate  stoker.  The  chain  grate  embodies  the  automatic 
principle  of  ash  disposal  more  completely  than  any  other  type, 
because  coal  is  fed  to  a  hopper  in  the  front  and  ash  discharged 
at  the  rear  automatically.  Both  of  these  functions  are  per- 
formed by  mechanical  means,  and  it  is  for  this  reason  that  the 
characteristic  curve  drops  off  so  rapidly  at  low  ratings. 

A  chain  grate  stoker  10  ft.  long,  and  feeding  a  fuel  bed  6 
inches  thick  to  burn  25  Ibs.  of  coal  per  square  ft.  of  grate 
surface  per  hour,  would  require  a  speed  of  the  grate  of  2~y2"  Per 
minute.  If  the  fire  were  carried  thinner,  a  corresponding 
increase  in  the  grate  speed  would  be  required,  but,  in  any  case, 
grate  speeds  of  over  4"  per  minute,  are  unusual,  and  the  wear 


SELECTION  OF  STOKER  EQUIPMENT 


159 


ISO 


250  300  35O  400  450  500  5SO  6OO  65O  700 

Lood  (Horse -pomr) 
Relation  between  Load  Efficiency  of  Boiler,  Furnace,  and  Grate. 


JW  350  400  450  500  550  6M  650  700  7fO 

Load  (Horse  power) 

Relation  between  Load  and  Efficiency  of  Boiler  and  Furnace,  Excluding  Grate. 


110 


/  o  6V7/J? 

ff  o  rfire 

IS  A  O'fire 

ff»  Sffirf 


\90 


1 80 


60 


200  &  JOO  350  400  45O  SOO  550  $00  650  700 

Load  (Horse- poner) 
Relation  between  Load  and  Efficiency  of  Boiler,  Excluding  Furnace  and  Grate. 

FIG.  69.— Efficiency  Chain  Grate  Stoker, 


160  MECHANICAL  STOKERS 

and  tear  on  the  stoker  at  grate  speeds  of  4"  or  over,  is  very  rapid. 
When  carrying  a  light  load  with  the  damper  partly  shut,  the 
last  two  or  three  feet  of  the  grate  surface  contains  only  a  small 
amount  of  fixed  carbon,  and  the  fire  is  more  or  less  dead  at 
this  part  of  the  grate  surface.  If,  in  this  condition,  it  becomes 
necessary  to  pick  up  a  load  suddenly,  and  the  boiler  dampers 
are  opened  wide,  it  is  only  a  few  minutes  until  the  small  amount 
of  coke  at  the  rear  of  the  furnace  is  burned  out,  and  there  is 
nothing  but  dead  ash  remaining. 

COAL  CONDITIONS 

The  coals  best  adapted  to  the  different  types  of  stokers, 
in  general,  are  pretty  well  defined  in  Chapter  IV.  For  all 
coals  having  coking  or  caking  tendency,  that  type  of  stoker 
so  designed  that  it  agitates  the  fuel  bed  and,  in  some  cases, 
uses  forced  draft,  is  most  suitable  for  these  fuels.  This  takes 
in  the  front  and  side  feed  stokers,  and  some  underfeed  types. 
Coking  coals  must  be  agitated  in  order  to  obtain  a  thorough 
air  distribution  through  the  fuel  bed.  If  coking  fuels  are  not 
agitated,  they  ball  up  into  large  masses  and  the  air  admission 
is  faulty  because  it  only  enters  the  fuel  bed  through  the  inter- 
stices between  the  large  masses.  The  result  is  that  a  large 
percentage  of  carbon  is  dumped  in  the  ash  pit  unconsumed. 
Most  of  the  eastern  coals  are  coking  or  caking  coals,  and  it 
is  for  this  reason  that  the  front  and  side  feed  and  underfeed 
stokers  have  been  very  successful  in  handling  this  kind  of 
coal.  There  are,  however,  some  eastern  coals  that  do  not  require 
agitation  when  being  burned,  and  in  using  this  grade  of  coal, 
the  stoker  that  agitates  the  fuel  bed  does  not  operate  success- 
fully. 

For  free  burning  coal,  the  chain  grate  type  of  stoker  is 
most  successful.  This  coal  does  not  require  agitation  and,  in 
fact,  when  it  is  agitated,  it  causes  trouble  on  account  of  its 
high  ash  content.  This  is  the  most  prominent  characteristic 
of  middle  western  coals,  and  it  is  for  this  reason  that  the 
chain  grate  stoker  has  been  so  successful  with  this  fuel.  The 
forced  draft  underfeed  stoker,  however,  is  burning  this  coal 
very  satisfactorily,  and  on  account  of  a  better  control  of  air 


SELECTION  OF  STOKER  EQUIPMENT 


161 


a  g 


a  a  s 


s  s 


K 
g 

w 


O        CD        i— i        CD 

TH          rH          <N          C^ 


9"  9   8   8   8    8   8   8 


00 


C^OiO"*!—  (Oi»—  iCO 


»O 


O 


162  MECHANICAL  STOKERS 

admission,  is  presenting  results,  in  overload  capacities,  that 
have  not  been  obtained  with  chain  grate  stokers. 

DRAFT  CONDITIONS 

The  draft  required  for  the  different  types  of  stokers  varies 
according  to  the  design.  With  underfeed  stokers,  the  air  for 
combustion  is  forced  through  the  fuel  bed  by  a  blower  so 
that  the  chimney  need  only  to  carry  away  the  products  of 
combustion.  Side  and  front  feed,  and  chain  grate  stokers, 
require  draft  in  the  furnace  and  over  the  fuel  bed,  necessary 
to  pull  the  air  through  the  fuel  bed  and  then  through  the 
boiler. 

Fig.  60  gives  the  pressure  required  in  the  wind  box  for 
multiple  inclined  underfeed  stokers.  Single  retort  underfeed 
stokers  require  from  V  to  5y2"  in  the  wind  box. 

Fig.  59  shows  the  draft  requirement  of  the  front  and  side 
feed  stokers  for  different  grades  of  coal.  The  minimum  draft 
for  this  stoker  should  not  be  less  than  .2"  in  the  furnace,  and 
the  application  of  this  stoker  to  old  boilers  should  not  be  con- 
sidered if  the  draft  required  to  burn  a  given  amount  of  coal 
is  not  available. 

For  chain  grate  stokers,  about  10  pounds  of  coal  can  be 
burned  per  square  foot  of  grate  surface  per  each  .1"  draft  in 
the  furnace.  There  should,  however,  be  a  minimum  of  .2" 
available  in  the  furnace  and  as  high  as  .6"  for  burning  about 
50  Ibs.  of  coal  per  square  foot  of  grate  area.  Fig.  71  gives  the 
draft  required  for  chain  grate  stokers  when  burning  different 
kinds  of  Illinois  coals. 

APPLICATION   CHARACTEIRSTICS 

When  stokers  are  applied  to  different  types  of  boilers,  the 
method  of  applying  them  differ.  Stokers  have  individual 
characteristics  that  must  be  considered  when  the  problem  of 
furnace  design  is  reached. 

Chain  Grate  Stoker. — The  chain  grate  stoker  requires  an  igni- 
tion arch  varying  from  3'-6"  to  6'  in  length,  depending  on  the  type 
of  boiler  and  the  grade  of  coal  to  be  burned,  the  lower  grade  coals 
requiring  the  longer  arches  for  their  ignition.  On  account 


SELECTION  OF  STOKER  EQUIPMENT 


163 


FIG.  70. — Suspended  Ignition  Arch. 


s 


s 


a 


•15 


7  o 

U  a  7' fire 

f  A  6'Fire 

S  •  %'firi> 


Z5O  300 


450  500  550 

Looct  (Horse-potter) 


600 


650 


FIG.  71.  —  Relation  between  Load  and  Draft  for  Chain  Grate  Stokers 
when  Burning  Illinois  Coals. 


164  MECHANICAL  STOKERS 

of  the  necessity  for  this  ignition  arch,  and  also  on  account  of 
the  length  of  the  stokers  usually  installed,  a  furnace  extension 
beyond  the  front  line  of  the  boiler  wall  is,  in  most  cases, 
required.  In  some  types  of  large  Stirling  boilers,  not  more 
than  18"  to  24"  is  required.  In  the  case  of  B.  &  W.  and  other 
vertically  baffled  boilers,  this  extension  varies  from  3'  to  5'. 
It  is  almost  necessary  to  provide  sufficient  space  in  front  of 
the  furnace  fronts,  to  completely  withdraw  the  stoker  from 
the  furnace.  It  is,  therefore,  necessary  to  provide  the  proper 
distance  in  the  firing  space  from  the  front  of  the  furnace  to 
the  building  wall  or  coal  bunker.  It  is  true  that  a  great  many 
chain  grate  stokers  are  installed  in  less  space  than  this,  but  it 
makes  repairs  difficult  and  tedious  when  the  stoker  cannot  be 
entirely  withdrawn  from  the  furnace  for  repairs. 

Another  characteristic  feature  of  the  chain  grate  which 
must  be  considered  is  that  considerable  coal  fired  sifts  through 
the  grates. 

Where  the  coal  contains  only  a  small  percentage  of  dust 
and  sufficient  amount  of  surface  moisture  to  make  the  coal 
stick  togther,  the  sifting  is  very  low.  On  the  other  hand, 
where  there  is  a  large  percentage  of  fine  dust  and  the  coal  is 
dry,  it  is  not  uncommon  to  get  considerable  siftings.  This 
falls  through  the  lower  part  of  the  chain  to  a  slab  or  pan 
located  slightly  below  the  floor.  Where  basement  construc- 
tion is  installed,  it  is  desirable  to  provide  hoppers  under  the 
stoker  chains  in  which  siftings  can  be  collected  and  returned 
by  means  of  conveyor,  or  otherwise,  to  the  coal  bunkers. 

Since  the  ash  is  deposited  at  the  rear  of  the  stoker,  it  is 
best  to  install  an  underground  pit  or  tunnel,  or  provide  a 
drag  to  pull  the  ash  to  the  front,  or  where  it  can  be  readily 
handled. 

To  protect  the  links  of  the  chain  at  the  rear  where  the 
ash  is  discharged  from  the  furnace,  an  overhanging  self-sup- 
ported firebrick  wall  is  provided,  or  a  firebrick  wall  supported 
on  a  water  cooled  pipe,  commonly  called  a  water  back.  This 
water  back,  or  firebrick  wall,  is  set  at  the  proper  height  above 
the  chain  to  permit  the  free  discharge  of  ash.  If  the  brick- 
work is  allowed  to  burn  off  or  become  broken  off,  a  consider- 
able amount  of  air  leaks  around  the  rear  of  the  sto"ker  into  the 


SELECTION  OF  STOKER  EQUIPMENT  165 

furnace.  If  the  water  back  is  supplied  with  water  from,  a  cold 
water  supply,  considerable  heat  is  lost  unless  this  water  is 
returned  to  a  hot  well.  To  overcome  this  loss,  it  is  best  to 
connect  the  water  back  into  the  boiler  circulation  and  under 
boiler  pressure.  This  construction  is  generally  recommended. 

Side  Feed  Stoker. — Side  feed  stokers  present  some  problems 
in  their  application  on  account  of  peculiar  construction  features. 
Since  the  coal  is  supplied  at  both  sides  of  the  stoker  and  the  coal 
magazine  runs  from  the  front  to  rear,  it  is  sometimes  difficult  to 
get  coal  to  the  rear  of  the  coal  magazine.  There  are  two  ways 
in  which  this  can  be  done.  Either  set  the  stoker  in  a  full 
" dutch  oven"  so  that  the  rear  of  the  magazine  comes  in  front 
of  the  front  wall  of  the  boiler,  or  have  the  boilers  set  singly 
so  that  the  coal  can  be  supplied  to  the  coal  magazine  from  the 
sides.  If  installations  do  not  provide  either  one  of  these  con- 
ditions, it  is  necessary  to  admit  coal  to  the  front  part  of  the 
coal  magazine  and  push  it  back  to  the  rear  with  a  bar,  or  by 
mechanical  means.  The  entire  grate  area  is  covered  by  a 
firebrick  arch,  either  supported  from  structural  work  or  sprung 
arch  shape  and  supported  by  skewbacks. 

Ash  disposal  is  not  a  serious  matter  since  a  grinder  is 
usually  located  at,  or  slightly  above,  the  floor  line  and  the 
ash  can  be  deposited  in  a  small  pit  and  raked  into  a  conveyor 
or  brought  out  on  the  floor  and  shoveled  into  wheel  barrows. 

The  siftings  of  coal  through  the  grate  surface  is  not  serious 
if  the  coal  contains  only  a  small  percentage  of  dust  and  the 
grate  bars  are  kept  in  first-class  condition. 

Since  a  full  "dutch  oven"  is  required,  this  means  a  long 
furnace  extension  and  provision  is  made  for  sufficient  space  in 
front  of  the  furnace  fronts  for  proper  operation. 

Front  Feed  Stoker. — Front  feed  stokers  are  built  on  a  con- 
siderable incline,  and  all  depend,  to  a  greater  extent,  on  the  force 
of  gravity  for  movement  of  the  fuel  bed.  Stokers  of  this  kind 
generally  employ  a  dumping  grate  and  also  use  a  fuel  guard  of 
some  kind  to  prevent  the  fuel  from  dropping  into  the  ash  pit  when 
the  dump  grates  are  dropped.  When  the  dump  grates  have 
finally  been  cleaned  and  are  brought  back  to  their  operating 
position,  the  guards  are  then  lowered  and  the  fuel  bed  moved 
downward. 


166  MECHANICAL  STOKERS 

This  type  of  stoker  does  not,  due  to  its  construction,  require 
an  extension  furnace,  but  in  obtaining  a  proper  furnace  design, 
an  extension  might  be  necessary. 

Ordinarily,  the  stoker  hopper  is  6'  or  7'  from  the  floor, 
and  if  coal  is  to  be  shoveled  into  the  hoppers  from  the  floor 
line,  the  stoker  front  is  depressed  so  that  the  top  of  the  hopper 
is  not  over  5'  from  the  floor  line. 

Ashes  can  be  raked  out  into  a  pit  at  the  front  of  the  stoker 
and  from  there  shoveled  into  wheel  barrows.  The  best  arrange- 
ment, however,  is  to  provide  hoppers  underneath  the  dump- 
ing grates  and  remove  the  ashes  by  cars  or  conveyors  in  a 
tunnel  below  the  boiler  room  floor. 

The  arch  construction  used  with  this  type  of  stoker  is 
very  important.  In  general,  it  may  be  said  that  the  ignition 
arch,  or  arch  placed  directly  over  the  front  part  of  the  stoker, 
should  be  not  less  than  5'  long  in  order  to  secure  proper  igni- 
tion of  the  particular  fuel  to  be  burned. 

Underfeed  Stoker. — The  multiple  retort  underfeed  stoker  is 
generally  applied  to  boilers  only  in  battery  or  single  settings  with 
alley-ways  at  least  8  ft.  wide,  so  that  access  to  the  furnace  can  be 
made  either  at  the  sides  or  at  the  rear. 

High  speed  forced  draft  fans  are  used  with  underfeed 
stokers,  and  these  are  located  in  a  place  where  they  are  access- 
ible and  easily  cleaned.  Care  is  generally  taken  not  to  place 
them  in  a  dark  corner  where  it  is  difficult  to  get  at  them. 

There  should  also  be  a  sufficient  supply  of  air  for  the  fans 
if  they  are  placed  in  basement.  Often  there  is  not  sufficient 
air  connections  Between  the  basement  and  the  outside  air  to 
provide  the  air  required. 

The  line  shafting  used  to  drive  the  stoker  is  also  best  placed 
above  the  boiler  room  floor,  if  possible,  so  that  attention  can 
be  given  to  oiling  and  maintaining  the  bearings. 

The  design  of  ash  pits  for  underfeed  stokers  requires  special 
consideration.  When  operating  at  high  ratings,  the  pits  must 
be  of  sufficient  volume  to  hold  a  reasonable  supply  of  ash  and 
refuse.  They  should  also  be  air-tight  and  provided  with  doors. 
When  the  stoker  is  cleaned,  if  open  pits  are  used,  spark  from 
the  dropping  ash  is  liable  to  injure  the  attendants.  The  doors 
on  the  ash  pits  should  be  of  sufficient  area  so  that  ash  can  be 


SELECTION  OF  STOKER  EQUIPMENT 


167 


taken  from  the  pits  without  difficulty.  An  underfeed  stoker 
requires  roomy  ash  pit  facilities,  and  is  not  adapted  to  applica- 
tions where  these  facilities  cannot  be  procured. 

On  account  of  the  high  temperatures  of  underfeed  furnaces, 
i.e.,  from  2500°  F.  to  3000°  F.,  the  very  best  firebrick  should 
be  used. 

Arches  are  not  generally  used  with  underfeed  stokers  when 
applied  to  boilers  where  the  proper  setting  can  be  obtained. 


FlG.  72. — Relation  between  Speed  of  Crank    Shaft  and  Amount  of  Coal 
Multiple  Retort  Underfeed  Stokers. 

Each  feeding  ram  will  displace  from  13  to  18  pounds  of  coal  per  stroke, 
depending  upon  the  size  and  kind  of  coal,  therefore  the  approximate  amount 
of  coal  used  can  be  determined  by  the  number  of  strokes  multiplied  by 
the  number  of  rams  or  retorts  per  stoker.  The  approximate  weights  are 
13  Ibs.  for  lignite  coal,  16  Ibs.  for  western  coal  and  18  Ibs.  for  eastern  coal. 

With  some  types  of  the  underfeed  stoker,  dumping  grates 
are  used  for  discharging  ash  and  clinker.  These  dumping 
grates  are  either  operated  by  hand;  mechanically  operated  by 


168  MECHANICAL  STOKERS 

steam  and  hydraulic  cylinders ;  or  an  electric  motor.  It  gener- 
ally requires  45  second  to  1%  minutes  to  operate  these  dump- 
ing grates,  depending  upon  the  type  installed  and  the  character 
of  the  fuel. 

For  higher  ratings  over  long  periods,  a  continuous  method 
of  ash  disposal  is  sometimes  used  in  the  form  of  rotary  clinker 
grinders.  These  have  been  applied  to  large  stokers  installed 
under  boilers  rated  at  1500  H.P.  and  above. 

Single  Retort  Underfeed  Stoker. — The  single  retort  underfeed 
stoker  with  side  grates  generally  does  not  require  ignition  arches 
unless  the  furnace  design  makes  such  a  connecting  medium 
advisable. 

With  reference  to  the  disposal  of  ashes,  inasmuch  as  the 
accumulation  of  ashes  is  at  the  sides  of  the  stoker,  the  same 
is  accessible  from  the  fire  doors  in  the  stoker  front.  The  dump^ 
ing  grates  are  manually  operated  from  the  front  and  the  ash 
discharged  into  the  low  ash  pits  at  the  side  of  the  stoker. 
This  ash  can  be  raked  to  the  front  of  the  stoker  and  shoveled 
in  the  wheel  barrows,  or  hoppers  can  be  provided  underneath 
the  dump  grates  and  the  ashes  taken  out  from  a  tunnel  below 
the  floor.  Almost  everything  in  connection  with  this  type 
of  stoker  is  accessible  from  the  front,  so  that  it  presents  no 
exact  requirements  insofar  as  accessibility  at  the  sides  or  the 
rear  of  the  stoker  is  concerned. 

FURNACE  DESIGN 

The  real  economy  in  boiler  rooms  comes  from  good  judg- 
ment used  in  making  the  combination  of  various  boiler  room 
apparatus  fulfill  their  proper  function.  The  individual  charac- 
teristics of  stokers  have  been  given  first,  because  the  design- 
ing of  furnaces  require  first  the  knowledge  of  the  individual 
features  of  stokers. 

Brickwork  and  Arches. — A  furnace  arch  is  made  of  refractory 
material  and  forms  the  roof  of  a  furnace,  or  it  is  an  arch  located 
within  the  furnace  for  the  purpose  of  aiding  combustion.  The 
combustion  or  ignition  arch  used  for  chain  grate  stokers  is  given 
considerable  attention,  because  it  has  been  found  that  the  burn- 
ing of  different  kinds  of  fuel  depend  to  a  great  extent  on  the 
length  of  these  arches  and  their  height  above  the  fuel  bed. 


SELECTION  OF  STOKER  EQUIPMENT  169 

In  the  chain  grate  type  of  stoker  complete  combustion  of 
coal  involves  series  of  events,  which  can  be  briefly  outlined 
as  follows: 

The  fresh  fuel,  containing  some  moisture,  enters  the  furnace 
where  it  is  subjected  to  the  influence  of  the  furnace  tempera- 
ture. The  moisture  is  first  driven  off,  which  is  accomplished 
by  the  time  the  temperature  of  the  particle  of  coal  under  con- 
sideration reaches  the  temperature  of  212°  F.,  and  as  the  tem- 
perature continues  to  increase,  the  volatile  constituents  begin 
to  come  off.  If  the  temperature  at  the  point  of  ignition  is 
sufficient,  these  burn  in  the  space  above,  and  approximately  as 
fast  as  they  are  distilled.  The  burning  of  this  portion  of  the 
fuel  forms  one  source  of  the  heat  required  for  the  ignition  of 
the  succeeding  charge  or  unit  of  fresh  coal. 

The  volatile  contents  are  not  all  driven  off  at  the  same 
temperature,  so  this  portion  of  the  process  continues  during 
an  appreciable  time  or  until  the  fuel  has  traveled  some  distance 
in  the  furnace. 

Temperature  readings  taken  at  the  point  of  ignition  indicate 
that  about  1100  deg.  must  be  obtained  before  the  volatile  will 
properly  ignite  and  1300  deg.  must  be  had  if  the  ignition  is 
to  be  prompt  and  bright. 

About  the  time  that  the  volatile  is  all  gone  the  particle 
of  coal  has  reached  a  quite  high  temperature  and  the  fixed  car- 
bon begins  to  burn. 

It  is  obvious  that  this  particular  stage  must  be  begun  as 
early  as  possible,  in  order  that  the  end  may  be  within  the 
time  limit  of  the  chain-grate  travel. 

Early  and  rapid  ignition,  therefore,  by  means  of  an  arch, 
becomes,  as  a  consequence,  vitally  necessary  to  the  proper 
completion  of  the  entire  process. 

With  an  arch  having  the  front  end  10  in.  from  the  surface 
of  the  grate  and  elevated  to  34  in.,  the  ignition  is  slow  and  the 
coal  will  pull  several  inches  away  from  the  gate  at  the  front 
of  the  grate  before  burning,  thus  losing  the  effect  of  a  portion 
of  the  grate  surface.  Many  experiments  in  suspending  the 
arch  for  the  purpose  of  getting  ignition  close  up  to  the  gate 
have  been  made,  and  found  that  by  having  the  arch  more 
nearly  horizontal  and  elevating  it  more  at  the  front,  the  coal 


170  MECHANICAL  STOKERS 

will  take  fire  right  at  the  edge  of  the  gate.  The  effect  from  the 
arch  suspended  in  this  manner  is  probably  due  to  decreasing 
the  opening  at  the  back  end  of  the  arch,  thus  holding  the 
fire  towards  the  front,  and  to  increasing  the  opening  at  the 
front  and  reducing  the  velocity  of  the  incoming  air  at  this 
point. 

In  addition  to  producing  quicker  ignition,  this  arch  adds 
to  the  capacity  of  the  boiler. 

An  ignition  arch  for  an  overfeed  stoker,  when  fuel  is 
fed  from  the  front,  is  made,  generally,  so  that  the  crown  of 
the  arch  is  horizontal.  The  length  of  the  arch  depends  con- 
siderably on  the  type  of  boiler  and  the  extension  of  the 
stoker  in  front  of  the  boiler.  For  Eastern  coals,  the  arch  is 
generally  made  about  4'  long — the  stoker  front  extending  in 
front  of  the  boiler  about  3'.  As  the  gases  move  along  the 
underside  of  this  arch,  it  is  well  to  have  sufficient  combustion 
space  above  the  rear  end  of  the  arch  so  that  there  will  be 
some  travel  for  the  gases  to  complete  combustion  before  they 
reach  the  cold  surface  of  the  boiler,  such  as  shown  in  Fig.  73. 
When  sufficient  height  cannot  be  obtained  and  the  proper 
distance  allowed  between  the  grates  and  the  cold  surface  of 
the  boiler,  a  longer  arch  is  used,  such  as  shown  in  Fig.  74. 
These  arches  are  sometimes  made  of  the  suspended  type  where 
special  firebrick  blocks  are  suspended  from  supporting  struc- 
ture over  the  stoker,  each  block  being  suspended  independently. 
In  this  design  arch,  the  difficulty  with  expansion  and  contraction 
of  the  refractory  material  have  no  deteriorating  effects.  The 
sprung  type  of  arch,  as  shown  in  Fig.  73,  is  often  used  where  the 
arch  is  made  of  standard  square  and  wedge  firebrick,  and  the 
skewbacks  supported  by  means  of  structural  angles  set  in  the  side 
walls.  Proper  buckstays  are  provided  so  that  the  skewback 
angles  cannot  move  outwards. 

First  grade  firebrick  is  generally  used  throughout  the 
furnace  setting.  Firebrick  in  the  side  walls  are  placed  9" 
headers,  18"  above  the  grate  line,  and  above  this  line  every 
fifth  course  is  a  stretcher  course  and  tied  in  with  the  red 
brick. 

The  furnace  arch,  which  is  also  an  aid  to  combustion,  is 
used  with  the  double  inclined  side  feed  stoker.  This  arch  is 


SELECTION  OF  STOKER  EQUIPMENT 


171 


either  made  of  the  flat  suspended  type  or  the  sprung  type 
supported  by  skewbacks. 

The  design  of  the  multiple  retort  underfeed  stoker  is  such 
that  ignition  or  furnace  arches  are  not  used.  Straight  front, 
side  and  bridge  walls  are  designed  wherever  it  is  possible  to  do 
so.  Considerable  trouble  has  been  experienced  with  the  brick 


FIG.  73. — Overfeed  Stoker  Applied  to  B.  &  W.  Boiler,  Showing  Extension  of 
Furnace  in  Front  of  Boiler. 


lining  of  the  front  or  bridge  walls  due  to  the  'fact  that  they 
bulge  out  and  eventually  fall  into  the  furnace.  The  front 
wall  immediately  over  the  throat  opening  of  the  stoker  is  con- 
structed as  shown  in  Fig.  94.  The  brickwork  in  the  furnace, 
in  general  cases,  is  made  of  9"  firebrick  lining,  and,  in  some 
cases,  13"  firebrick  is  used. 

If  it  is  necessary  to  use  arches,  they  are  either  of  the 
sprung  or  suspended  type,  and,  where  very  high  ratings  are 
expected,  arrangements  are  made  to  place  a  slight  pressure 
over  the  arches  so  that  air  is  pulled  from  the  outside  into  the 


172 


MECHANICAL  STOKERS 


furnace  instead  of  having  the  flames  lap  backward  between 
the  blocks  of  the  arches. 

Location  of  Observation  Doors. — It  is  always  necessary  to 
obtain  complete  observation  of  furnace  conditions,  so  that,  with 
the  multiple  retort  underfeed,  the  front  feed,  and  chain  grate 
types  of  stokers,  it  should  be  made  possible  to  install  one  door  at 


FIG.  74. — Stoker  and  Boiler  Application,  Showing  Distance  Required  between 
Dumping  Grates  and  Lower  End  of  Tubes. 

least  in  each  side  of  the  furnace.  Stoker  installations,  where  it 
is  not  possible  to  do  this,  make  operation  very  difficult.  One  door 
should  be  placed  near  the  throat  opening  where  the  fuel  enters 
the  furnace,  and  the  other  placed  near  the  ash  disposal  mechan- 
ism. With  large  furnaces,  observation  door  (Fig.  85),  located 
in  the  bridge  wall,  are  very  convenient. 

Clinker  ing  of  Coal. — The  formation  of  clinker  at  the  side 
walls  of  furnaces  and  the  difficulty  of  removing  them  when  formed 


SELECTION  OF  STOKER  EQUIPMENT 


173 


is  one  of  the  most  troublesome  experiences  of  stoker  operation, 
besides  being  a  serious  factor  in  high  maintenance  cost  on  settings. 
To  reduce  trouble  of  this  kind,  cast-iron  air  boxes  have  been  built 
into  the  side  walls  of  the  furnace.  A  4-in.  pipe  is  connected 
to  one  of  the  air  boxes,  which  are  all  connected  together.  Into 
this  4-in.  pipe  a  1%-in.  steam  line  is  led  to  act  as  an  inspirator; 
the  steam  jet,  discharging  into  the  4-in.  pipe,  draws  air  into 


Part*  Section  -through  Furnace   Walls 
A,B,0,D,E&F 


P/an 


\ 

* 

— 

&  <p 

<~g£~~9> 

$ 

r 

t 

«r,o 

Siafe  Frorrf- 

Detail  of  Block 


FIG.  75. — Application  and  Details  of  Drake  Furnace  Blocks. 

the  open  end  of  that  pipe.    Three-quarter  inch  holes  are  drilled 
into  the  side  of  the  box  facing  the  furnace. 

To  prevent  the  formation  of  clinkers  on  the  side  walls  of 
furnaces  a  method  has  been  used  consisting  of  an  arrange- 
ment of  firebrick  in  the  furnace  walls  whereby  air  is  ad- 
mitted into  the  fuel  bed  between  the  bricks  of  the  side  set- 
ting. By  supplying  forced  draft  through  these  air  passes,  the 
face  of  the  brick  in  the  furnace  wall  is  kept  comparatively 
cool,  the  idea  being  that  the  clinker  will  not  then  adhere  to 


174 


MECHANICAL  STOKERS 


the  brick  surface.  The  combustion  at  these  points  is  stimulated 
and  increased. 

Where  forced  draft  is  applied  to  the  fires,  a  branch  conduit 
is  connected  to  the  air  passage  back  of  the  side  walls  and  the 
same  forced  draft  is  applied  through  the  brick  spaces  in  the 
side  wall. 

The  circulation  of  air  passing  directly  into  the  fire  from 
the  furnace  walls  supplies  draft  at  a  point  where  it  seems  to 
be  needed,  A  damper  or  valve  operated  from  the  outside  con- 


FIG.  76. — Interior  View  of  Stoker,  Showing  Side  Plates  for  Prevention  of 
Clinker  Adhesion  to  Side-walls. 

trols  the  amount  of  air  thus  supplied.  No  more  forced  draft 
is  used  than  that  supplied  to  the  fire  through  the  grate. 

In  some  cases  steam  jets  are  installed  along  the  side  tuyere 
boxes  of  underfeed  stokers.  This  steam  tends  to  rot  the  clinker 
so  that  it  is  more  easily  removed  from  the  side  walls. 

Specially  designed  firebrick  blocks  are  sometimes  installed 
in  the  side  walls  of  the  furnace  and  arranged  for  air  to  enter 
the  furnace  through  these  blocks.  This  system  is  shown  in 
Fig.  75. 

Where  the  furnace  will  allow  it,  very  often  side  plates  are 


SELECTION  OF  STOKER  EQUIPMENT 


175 


used  next  to  the  retorts  of  the  stoker,  as  shown  in  Fig.  76, 
which  serves  to  keep  the  fuel  bed  away  from  the  walls. 

The  accumulation  of  clinker  on  the  bridge  walls  of  under- 
feed stoker  installations  is  partly  overcome  in  the  design.  In 
one  case  the  stoker  is  equipped  with  dump  grate  operating 
mechanism  that  is  designed  to  knock  loose  the  clinker  adhesion 
from  the  bridge  wall.  This  method  as  used,  is  shown  in  Fig. 
77.  High  and  low  pressure  water  backs  are  also  sometimes 
placed  in  the  bridge  wall  to  overcome  the  adhesion  of  clinker. 


Second  operation  with  Taylor 
Power  Dump.  Dump  plate  is 
raised  to  loosen  the  clinker  masses 
on  lower  grate,  and  to  knock  off 
slag  adhesions  on  bridge-wall.  If 
desired,  it  may  be  dropped  a 
second  time  to  get  rid  of  them. 


f\ 


FIG.  77. — Dump  Grate  Operation  Designed  to  Loosen  the  Clinker  Adhesion 

from  Bridge-wall. 

!••          •'••'  'f:£f  '* 

Another  scheme  is  shown  in  Fig.  78,  where  a  series  of  tubes 
is  set  into  the  lower  part  of  the  bridge  wall  to  assist  in  draw- 
ing heat  from  the  ashes  and  preventing  the  clinker  formation. 
These  tubes  are  made  to  extend  out  through  the  bridge  wall  and 
connect  to  headers. 

In  some  designs  of  the  front  inclined  overfeed  type  of 
stoker,  where  clinkering  coal  is  used,  the  exhaust  steam  from 
the  stoker  engines  is  used  in  connection  with  steam  jets  placed 
under  the  grates  similar  to  that  shown  in  Fig.  79.  The  small 
holes  in  the  steam  outlets  are  directed  downward  and  the 


176  •      MECHANICAL  STOKERS 

draft  pulling  the  steam  up  through  the  fuel  bed  tends  to 
moisten  and  break  up  the  clinker  formation. 

The  dump  grates  of  this  type  of  stoker  are  supported  in 
such  a  manner  that,  when  the  ash  and  refuse  is  dumped,  the 
rear  part  of  the  dump  grate  travels  through  an  arc  and 
tends  to  pull  the  clinker  away  from  the  bridge  wall  (Fig.  80). 


•piG.  78. — Water  Tubes  Set  in  Lower  Half  of  Bridge-wall  to  Prevent  Clinker 
Formation  on  the  Wall. 

Air  Over  the  Fire. — The  Bureau  of  Mines,  U.  S.  Geological 
Survey,  have  shown  that  air  taken  through  ordinary  hand-fired 
grates  is  completely  used  up  and  the  oxygen  in  the  air  combined 
with  the  carbon  in  the  coal  before  it  has  a  chance  to  get  entirely 
through  the  fuel  bed.  Although  this  is  not  exactly  similar  to 
stoker  fired  furnace  operation,  on  account  of  the  more  even 
distribution  of  air,  nevertheless  with  certain  high  volatile  coals, 


SELECTION  OF  STOKER  EQUIPMENT 


177 


even  with  some  stokers,  air  over  the  fire  is  necessary.  This 
has  been  accomplished  in  many  cases  by  taking  air  from  a 
channel  built  in  the  front  wall  of  the  stoker,  as  shown  in 
Fig.  81. 


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v  X^-  '      J       ' 

O-TV 


FIG.  79. — Steam  Jets  Placed  under  Stoker  Grates  to  Assist  in  Loosening 

Clinker  Formation. 

Mixture  of  Gases  in  the  Furnace. — In  order  to  obtain  an 
intermingling  of  gases  in  the  furnace  to  complete  combustion  at 
the  earliest  possible  time,  steam  jets  are  installed  at  the  front 
of  the  front  inclined  overfeed  stoker  and  three  or  four  jets  of 
steam  sprayed  into  the  volatile  gases,  as  they  are  distilled  from 
the  fuel  bed  (Fig.  82).  These  jets  of  steam  tend  to  mix  the 
gases  with  air  coming  down  through  the  channels  in  the  front 


178  MECHANICAL  STOKERS 

of  the  stoker,  and  is  so  designed  to  mix  the  gases  in  the  furnace 
and  complete  combustion  before  the  gases  reach  the  boiler  tubes. 
Ash  Pit  Construction. — The  design  and  construction  of  ash 
pits  for  underfeed  stokers  is  very  important  so  that  the  entire 
installation  will  not  be  limited  as  to  the  amount  of  ash  and  refuse 
that  can  be  stored  or  removed  in  a  certain  time.  For  proper  size 
of  ash  pits,  etc.,  basements  under  underfeed  stokers  should  be  at 
least  20  ft.  high.  This  will  then  give  ample  room  for  installation 


FIG.  80. — Dumping  Grate  and  Guard  of  Overfeed   Front  Inclined  Stoker, 
Showing  Method  of  Breaking  off  Clinker  Adhesion  to  Bridge-wall. 

of  fans,  ash  hoppers,  ash  cars  and  air  ducts.     Such  a  basement 
is  shown  in  Fig.  89. 

Draft. — The  maintenance  of  brickwork  in  an  underfeed 
furnace  becomes  increasingly  difficult  with  low  draft  pressure. 
The  result  of  low  draft  in  an  underfeed  furnace  is  to  start  a 
puddling  condition  in  the  furnace  which  directs  the  flames  back 
into  the  cracks  in  the  brickwork.  In  one  installation,  experi- 
ments demonstrated  that,  by  pulling  the  gases  through  the 
boiler  at  a  higher  velocity,  the  capacity  of  the  boiler  was 
increased  from  80,000  Ibs.  of  steam  to  120,000  Ibs  of  steam. 
In  the  first  instant,  considerable  trouble  was  experienced  with 
firebrick,  and  after  the  draft  was  increased  and  the  gases 


SELECTION  OF  STOKER  EQUIPMENT  179 

pulled  through  the  boiler  and  from  the  furnace  at  a  higher 
velocity,  this  trouble  ceased. 


FIG.  81. — Magazine  Construction,  Showing   Method  of  Admitting  Air  over 
the  Fire  in  "  V  "  Type  Overfeed  Stoker. 

COMBUSTION   SPACE 

Underfeed  Stokers. — Considerable  effect  is  realized  from  the 
incandescent  fuel  bed  of  an  underfeed  stoker  fire.  For  the  same 
rate  of  combustion,  the  same  combustion  space  is  not  required  for 
this  type  of  stoker,  compared  with  other  types.  On  account,  how- 


180 


MECHANICAL  STOKERS 


SELECTION  OF  STOKER  EQUIPMENT  181 

ever,  of  the  fact  that  higher  rates  of  combustion  are  possible, 
and  generally  used  with  this  stoker,  the  combustion  space 
should  be  as  much  as  is  required  for  other  types  of  stokers. 
For  Eastern  Bituminous  coal  not  less  than  10  cubic  ft.  com- 
bustion space  should  be  used  per  sq.  ft.  of  grate  surface,  when 
a  rate  of  combustion  does  not  exceed  75  Ibs.  of  coal  per  sq. 
ft.  of  grate  surface  per  hour.  For  Pittsburgh,  Illinois  and 
Middle  Western  coals,  at  least  12  cubic  ft.  of  combustion  space 
should  be  used.  This  would  require  setting  B.  &  W.,  Heine 
and  similar  types  of  boilers,  from  10'  to  14'  from  the  floor 
line  to  the  front  header.  When  this  stoker  is  installed  in 
connection  with  the  Stirling,  Connelly,  Ladd,  Erie  City  or 
similar  types  of  boilers,  the  dump  grates  of  the  stoker  should 
be  not  less  than  7'  from  the  tubes  where  they  enter  the  lower 
drum. 

Front  and  Side  Feed  Stokers. — For  Eastern  Bituminous  coal, 
the  front  inclined  overfeed  stoker,  should  be  set  with  at  least  an 
extension  of  4'  in  front  of  the  boiler,  with  an  arch  at  least  5'  6" 
long.  When  this  stoker  is  combined  with  the  B.  &  W.,  Heine  and 
similar  types  of  boilers,  the  front  headers  should  be  set  11'  from 
the  floor  line.  For  Pittsburgh,  Illinois  and  other  high  volatile 
coals,  this  stoker  should  have  an  arch  at  least  T  long  with 
the  stoker  extended  5'  in  front  of  the  boiler.  The  front 
header  of  the  B.  &  W.,  Heine  or  similar  type  boiler  should  be 
set  at  least  12'  from  the  floor  line  for  this  stoker.  For  the 
Stirling,  Connelly,  Wicks  and  similar  types  of  boilers,  the 
dump  grates  of  this  stoker  should  be  at  least  T  from  the  tubes 
where  they  enter  the  lower  drum. 

For  the  side  feed  stokers,  on  account  of  the  Dutch  Oven 
extension,  considerable  combustion  space  is  obtained  by  the 
mere  construction  of  this  stoker,  but  there  should  be  at  least 
6'  from  the  rear  grates  of  this  stoker  to  any  boiler  tubes.  The 
reason  for  this,  is  that  there  is  green  coal  fed  to  that  part  of 
the  grate  surface  nearest  to  the  boiler  and  there  should  be 
sufficient  flame  travel  from  that  part  of  the  grate  surface  to 
the  tubes  of  the  boiler. 

Chain  Grate  Stokers. — Chain  grate  stokers  have  been  used 
more  than  any  other  in  connection  with  Middle  West  High  Vola- 
tile coals  and  for  this  reason,  considerable  development  work  has 


182  MECHANICAL  STOKERS 

been  done  in  connection  with  the  combustion  space  when  this 
stoker  is  applied  to  different  types  of  boilers.  As  previously  out- 
lined, the  arch  in  connection  with  the  furnace  design  of  this  stoker 
has  considerable  effect  on  combustion,  and  naturally  this  effects 
the  type  of  combustion  chamber  required.  When  burning 
Indiana  coals  the  furnace  volume  should  be  from  11  to  15 
cubic  ft.  per  sq.  ft  of  grate  surface.  Arches  varying  from 
4'  to  6'  in  length,  are  also  used  as  a  factor  in  designing  the 
combustion  space.  When  burning  Iowa  coals,  the  combustion 
space  and  length  of  arch,  is  again  of  great  importance,  since 
the  poorer  grades  of  coal,  as  hereinbefore  mentioned,  are  of  low 
heat  value  and  therefore  difficult  to  burn.  These  coals  require 
an  arch  covering  Yz  of  the  entire  grate  surface,  and  there 
should  be  provided  at  least  12'  between  tubes  of  a  B.  &  W., 
Heine  or  similar  type  of  boiler  and  the  grate  surface,  and  at 
least  7'  between  the  rear  of  the  grate  and  the  tubes  of  a 
Stirling,  Connelly  or  similar  type  boiler,  where  they  enter  the 
lower  drum.  When  this  stoker  is  used  for  burning  lignite 
fuel,  the  design  of  the  arch  is  again  of  utmost  importance, 
because  these  coals  contain  from  25  to  40%  moisture  and  it  is 
necessary  to  provide  a  drying  out  process  before  they  can  be 
burned.  There  should  be  provided  at  least  10'  between  the 
boiler  tubes  of  a  B.  &  W.,  Heine  or  similar  type  boiler  and 
the  grate,  and  at  least  7'  when  this  stoker  is  applied  to  the 
Stirling,  Connelly,  or  similar  type  of  boiler  between  the  rear 
of  the  grate  and  the  tubes  where  they  enter  the  bottom  drums. 

THE  FLOW  OF  HEAT  THROUGH  FURNACE  WALLS 

After  careful  investigations  and  research  work,  by  the 
Bureau  of  Mines,  U.  S.  Geological  Survey,  bulletin  number  8, 
gives  the  following  conclusions  regarding  the  heat  flow  through 
furnace  walls. 

In  the  case  of  the  furnace  wall,  where  the  quantity  of 
heat  passing  between  any  two  planes  which  are  parallel  to 
the  surfaces  of  the  walls  is  the  same,  the  temperature  difference 
between  any  two  planes  indicates  the  resistance  which  the 
material  or  space  between  the  two  planes  offers  to  the  flow 
of  heat.  For  example,  if  the  temperature  difference  between 


SELECTION  OF  STOKER  EQUIPMENT  183 

the  faces  of  the  firebrick  wall  is  high,  it  may  be  said  that  the 
resistance  to  the  heat  flow  of  the  firebrick  wall  is  high;  or, 
if  the  temperature  difference  between  the  two  surfaces  on  each 
side  of  the  air  space  is  low,  it  may  be  inferred  that  the  resist- 
ance to  the  heat  passage  across  the  air  space  is  low.  Thus  it 
is  possible  to  rely  on  the  temperature  difference  as  being  a 
true  indicator  of  "high  or  low  resistance  to  heat  flow  between 
any  two  planes  which  are  parallel  to  the  surface  of  the  wall. 
With  this  knowledge  the  reader  can  turn  to  the  charts  and 
study  the  resistance  of  the  firebrick,  the  air  space,  the  asbestos 
layer,  and  the  common  brick,  and  the  relative  value  of  these 
materials  as  heat  insulators  in  the  construction  of  furnace 
walls. 

Fig.  83  gives  the  temperature  drops  through  the  side  wall 
as  recorded  by  the  set  of  couples  placed  at  &,  and  through  the 
roof  as  recorded  by  the  set  of  couples  placed  at  c.  At  the 
foot  of  the  figure  is  shown  diagrammatically  the  thickness  of 
the  side  wall,  and  at  the  top  of  the  figure  is  shown  thickness 
of  the  roof ;  in  each  case  the  measurements  of  thickness  are 
used  as  abscissae  in  the  chart.  The  temperatures  at  the  various 
points  are  platted  as  ordinates.  The  figure  shows  three  tem- 
perature gradients  or  drops  through  the  wall  and  through  the 
roof,  one  at  11  a.  m.,  April  12,  when  the  test  was  started,  one 
at  4  p.  m.  the  same  day,  and  one  at  2  p.  m.  the  next  day.  The 
first  two  gradients  give  the  relation  of  the  temperatures  before 
the  equilibrium  is  reached,  and  are  interesting  only  when  com- 
pared with  one  another  to  show  how  the  temperatures  change 
with  respect  to  each  other  while  the  walls  are  being  heated. 
The  last  gradient  represents  the  equilibrium  and  is  most  of 
interest. 

The  striking  feature  concerning  the  side  wall  thermocouples, 
set  6,  is  the  large  temperature  drop  through  the  firebrick 
wall,  the  very  small  drop  through  the  air  space,  and,  again, 
the  large  temperature  drop  through  the  common  brick  wall. 
These  drops  plainly  indicate  that  the  resistance  to  heat  pas- 
sage of  the  air  space  is  very  low  compared  with  that  of  either 
brick  wall,  only  about  one-fourth  as  much.  The  last  tempera- 
ture gradient  through  the  roof,  as  given  by  the  set  of  thermo- 
couples c,  show  a  rather  low  temperature  drop  through  the 


184 


MECHANICAL  STOKERS 


THICKNESS  OP  ARCH  ROOF,  INCHES. 
8         10        12         14 


2  4  6  8          10          12         14          16         18         20 

THICKNESS  OF  SIDE  WALL,  INCHES. 
FIG.  83. — Temperature  Drops  through  Furnace  Walls. 


SELECTION  OF  STOKER  EQUIPMENT  185 

firebrick,  a  high  drop  through  the  1  inch  layer  of  asbestos, 
and  a  rather  small  drop  through  the  common  brick.  These 
temperature  drops  indicate  that  the  resistance  to  heat  flow  of 
the  1  inch  asbestos  layer  is  higher  than  that  of  7  inches  of 
firebrick.  By  comparing  the  last  gradient  of  couples-  b  with 
that  of  couples  c,  it  is  easy  to  see  that  1  inch  of  asbestos  is 
much  more  effective  as  a  heat  insulator  under  the  existing  con- 
ditions than  a  2  inch  air  space.  Although  the  total  thickness 
of  the  roof  is  5  inches  less  than  that  of  the  side  wall,  a  smaller 
quantity  of  heat  per  sq.  ft.  is  lost  through  it  than  through 
the  side  wall. 

The  results  of  the  investigation  as  outlined  in  this  bulletin 
justify  the  following  conclusions: 

In  furnace  construction  a  solid  wall  is  a  better  heat  insulator 
than  a  wall  of  the  same  total  thickness  containing  an  air  space. 
This  statement  is  particulary  true  if  the  air  space  is  close  to 
the  furnace  side  of  the  wall,  and  if  the  furnace  is  operated  at 
high  temperatures.  If  it  is  desirable  in  furnace  construction 
to  build  the  walls  in  two  parts,  so  as  to  prevent  cracks  being 
formed  by  the  expansion  of  the  brickwork  on  the  furnace 
side  of  the  walls,  it  is  preferable  to  fill  the  space  between  the 
two  walls  with  some  " solid'*  (not  firm,  but  loose)  insulating 
material.  Any  such  easily  obtainable  materials  as  ash,  crushed 
brick,  or  sand  offer  higher  resistance  to  heat  flow  through  the 
walls  than  an  air  space.  Furthermore,  any  such  loose  material 
by  its  plasticity  reduces  air  leakage,  which  is  an  important 
feature  deserving  consideration. 


CHAPTER  VII 

STOKER  EQUIPMENT  OF  MODERN  STEAM  POWER 
STATIONS 

A  study  of  the  fuel  burning  equipment  of  the  most  modern 
plants  will  show  that  every  consideration  is  being  given  to 
those  details  which  provide  for  a  balancing  of  the  economic 
results  that  come  from  a  careful  selection  of  equipment,  good 
supervision  and  correct  operation.  It  will  be  found  that 
elaborate  means  are  being  provided  so  that  the  boiler  room 
organization  can  do  things  easily.  It  is  no  longer  necessary 
for  firemen  to  climb  ladders  and  crawl  over  the  boiler  tops  to 
change  the  position  of  dampers,  although  such  methods  are 
still  common  in  many  old  plants.  Mechanisms  are  being  placed 
at  the  hands  of  the  operators  so  that  it  is  not  necessary  for 
them  to  go  to  inconvenient  places  in  order  to  control  operating 
conditions. 

The  most  generally  used  fuel  burning  equipment  in  the 
modern  stations  is  the  "inclined  multiple  retort"  underfeed 
and  the  "chain  grate"  stoker,  designed  for  large  boiler  units, 
ranging  from  1200  to  1500  H.P.  up.  A  number  of  boilers  contain- 
ing 12,500  sq.  ft.  of  heating  surface  have  been  used  and  are  furnish- 
ing steam  for  7000  to  8000  kw.  in  the  prime  mover.  It  is  not 
at  all  improbable  that  this  unit  will  be  further  developed  to 
furnish  steam  for  at  least  10,000  kw.,  in  the  prime  mover  for 
continuous  operation.  These  units  are  set  singly  with  large 
alley  ways  between  each  setting,  so  that  the  boiler  and  fuel- 
burning  equipment  are  accessible  on  all  sides.  The  stokers  are 
designed  for  a  flexible  operation  of  50  to  300  per  cent  rating. 
Clinker  grinders  are  used  in  a  number  of  cases  for  discharging 
the  ash  and  refuse  automatically  from  underfeed  stokers. 

The  following  brief  description  of  the  fuel-burning  equip- 
ment installed  in  a  number  of  modern  power  stations  covers 

186 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS  187 

a  wide  range  in  the  character  of  load  and  fuel  used.  Some 
of  these  plants  are  completely  new  stations,  while  others  are 
extensions  to  old  stations,  and  still  others  are  old  stations  in 
which  inadequate  fuel-burning  equipment  has  been  replaced 
by  more  modern  equipment. 


EDISON  ELECTRIC  ILLUMINATING  COMPANY. 
Boston,  Mass. 

This  company  replaced  old  fuel-burning  equipment  under 
eight  512  horse  power  boilers  with  inclined  underfeed  stokers. 
One  of  these  stokers  was  equipped  with  a  clinker  grinder,  the 
idea  being  to  try  this  out  under  regular  operating  conditions 
and  with  the  fuel  available,  this  being  a  part  of  a  study  for 
the  new  extension  to  the  station.  Although  the  stoker  was  of 
small  size  (five  retorts),  the  clinker  grinder  operated  satis- 
factorily and  it  was  decided  to  use  this  design  in  connection 
with  the  equipment  for  the  new  extension  consisting  of  four 
cross-drum  boilers,  42  sections  wide,  14  tubes  high  and  18  ft. 
long,  rated  at  1232  H.P.,  at  300  Ibs.  gauge  pressure  equipped 
with  a  superheater  designed  to  give  150°  superheat. 

These  stokers,  as  shown  in  Fig.  84,  are  of  the  Westing- 
house  underfeed  type,  having  13  retorts  installed  under  the 
back  end  of  the  boiler,  under  the  mud  drums,  and  equipped 
with  rotary  clinker  grinders  (Fig.  55)  for  removing  the  ash 
and  clinker  continuously.  The  stoker  drives  (Fig.  86)  are 
divided  with  not  over  four  retorts  to  a  motor;  also,  the  wind 
boxes  and  dampers  are  so  arranged  that  they  can  be  con- 
trolled on  the  same  basis,  this  provision  being  made  so  as  to 
give  a  complete  control  of  coal  and  air  across  the  entire  furnace 
width. 

The  coal  used  is  New  River,  of  approximately  the  following 
analysis : 

Fixed  carbon   73.50 

Volatile 20.75 

Ash  5.75 

Moisture    3.25 

Sulphur 1.05 

B.T.U 14,700 


188 


MECHANICAL  STOKERS 


The  average  per  cent  of  combustible  in  the  ash  and  refuse 
is  not  to  exceed  15  per  cent.  The  stoker  equipment,  when 
supplied  with  the  above  fuel,  is  designed  to  develop  300  per 
cent  of  normal  rating  of  the  boilers  for  periods  of  short  dura- 
tion. 


FIG.  84. — Boston  Edison  Stoker  Setting,  Showing  Clinker  Grinder  and  Furnace 
Control  Mechanism  Located  at  the  Front  of  the  Boiler. 


Doors  are  placed  in  the  bridge  wall  and  all  controlling 
mechanism  placed  at  the  end  opposite  the  stoker  (Fig.  84)  so 
that  when  the  operator  views  the  furnace  fires  through  the 
bridge  wall  doors,  he  will  have  the  controlling  mechanism  at 
hand. 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS     189 

UNITED  ELECTRIC  LIGHT  AND  POWER  COMPANY,  HELL  GATE 

New  York  City 

The   fuel   burning   equipment   at   this   station   consists   of 
twelve  28-retort,  type  AA7  Taylor  Stokers,  each  installed  under 


FIG.  85. — Interior  View  of  Boston  Edison  Furnace,  Showing  Clinker  Grinder 
and  Front-wall  Construction. 

1890  horse  power  boilers.  This  type  of  Taylor  Stoker  has 
seventeen  tuyeres  and  two  feeding  rams  per  retort.  It  is  pro- 
vided with  double  rotary  clinker  crushers  of  the  same  general 
design  as  those  used  on  the  Taylor  Stokers  at  the  Delray  Plant 
of  the  Detroit  Edison  Company. 


190 


MECHANICAL  STOKERS 


The  combined  boiler  and  furnace  efficiencies  of  these  units 
will  range  from  76%  at  150%  of  rating  to  approximately  63% 
at  360%  of  rating,  when  burning  Eastern  Bituminous  coal  of 
approximately  13,000  B.T.U.  per  pound  as  fired. 


FIG.  86. — Front  View  of  Boston  Edison  Stokers,  Showing  Motor  Driving 

Equipment. 

The  ashes  will  be  discharged  directly  into  a  flume  from 
which  they  will  be  removed  by  water. 

This  stoker  and  boiler  arrangement  is  shown  in  Fig.  87. 


PUBLIC   SERVICE  ELECTRIC   COMPANY, 
Newark,  NJ. 

The  " Essex"  station  of  the  above  company,  located  on  the 
Passaic  Eiver  about  two  and  one-half  miles  from  Newark,  N.  J., 
was  placed  in  operation  in  1915.  The  boiler  room  equipment 
consists  of  B.  &  W.  Cross  Drum  boilers  of  1373  horse  power  each 
with  an  operating  pressure  of  225  pounds  and  superheat  of 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    191 


FIG.  87.— Setting  of  Stokers  and  Boilers  of  the  Hell  Gate  Plant,  United 
Electric  Light  &  Power  Company. 


192 


MECHANICAL  STOKERS 


100°  F.  Each  boiler  is  equipped  with  16  retorts,  Riley  Under- 
feed stokers,  in  a  Duplex  setting  (Fig.  88).  The  stoker  equip- 
ment was  designed  to  burn  up  to  15,000  pounds  per  hour,  when 
burning  Pittsburgh  coal  of  about  13,500  B.T.U.  (as  fired)  and 
to  develop  about  500%  and  600%  of  boiler  rating  for  short 
periods. 


FIG.  88. — Stoker  Installation  Public  Service  Electric,  Showing  Stoker 
Operating  Board. 


PHILADELPHIA  ELECTRIC  COMPANY, 

Philadelphia,  Pa. 
Delaware  Avenue  Station 

At  this  Station  special  type  Stirling  boilers  of  1508  horse 
power  each  are  equipped  with  fifteen  retort  type  BAT  Taylor 
Stokers  (Fig.  89).  These  stokers  have  twenty-two  tuyeres  and 
three  rams  per  retort.  They  are  provided  with  double  roll 
clinker  crushers  and  are  capable  of  operating  the  boilers  up 
to  330%  of  rating,  when  burning  Eastern  coal  of  approximately 
13,800  B.T.U.  per  pound  as  fired,  and  having  the  following 
proximate  analysis: 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    193 

Moisture 2.3 

Volatile 22.5 

Fixed  Carbon 65.0 

Ash  10.2 

Sulphur  1.83 


FIG.  89. — Setting  of  Stokers  at  Philadelphia  Electric  Delaware  Avenue  Station. 

The  efficiencies  of  the  combined  unit  (boiler,  furnace  and 
economizer)  will  range  from  82%  at  rating  to  70%  at  250% 
rating. 


194 


MECHANICAL  STOKERS 


CONSOLIDATED  GAS  AND  ELECTRIC  CO., 
Baltimore,  Md. 

At  the  Westport  Station  of  the  Consolidated  Gas  &  Electric 
Company,  Baltimore,  11-retort  22-tuyere  type  Taylor  Stokers, 
similar  to  Fig.  91,  are  in  service  under  1074  horse  power  Water- 
tube  boilers. 


FIG.  90. — Interior  View  Delaware  Avenue  Station,   Philadelphia  Electric, 
Showing  Stoker  Operating  Board. 

These  stokers  are  of  the  22-tuyere  type,  and  are  equipped 
with  power  operated  dump  plates,  which  are  designed  to  swing 
above  the  horizontal  to  clear  the  bridge  wall  of  clinkers. 

This  installation  is  designed  to  burn  Eastern  coals  of  the 
following  characteristics : 

Moisture   2.00 

Volatile  18.00 

Fixed   Carbon    62.00 

Ash    16.00 

Sulphur    2.00 

B.T.U.  per  pound  (as  fired) 12,600 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    195 

This  installation  will  burn  sufficient  of  the  above  coal  to 
develop  350%  of  boiler  rating. 

The  efficiency  through  the  normal  range  will  run  from  ap- 
proximately 72%  at  250%  of  rating  to  77%  at  150%  of  rating. 


FIG.  91. — Setting  of  EdgeMoor  Boilers  and  Taylor  Stokers  at  Consolidated 
Gas  &  Electric  Company. 

BUFFALO   GENERAL  ELECTRIC  COMPANY, 
Buffalo,  N.  Y. 

The  Niagara  Eiver  station  of  this  company  was  built  in 
1916  with  an  ultimate  total  capacity  of  200,000  K.W.  The  fuel 
burning  equipment  consists  of  ten  1140  horse  power  cross  drum 
boilers  of  the  B.  &  W.  type.  The  stoker  equipment  was  designed 
for  burning  Pittsburgh  coals  of  the  following  analysis: 


196  MECHANICAL  STOKERS 

Moisture   3.00 

Ash    10.00 

Sulphur  2.00 

B.T.U.  (as  fired)    13,500 

To  eliminate,  as  much  as  possible,  the  formation  of  clinkers, 
air  boxes  were  designed  for  installation  in  the  sidewalls.  Each 
boiler  is  served  by  two  15-retorts,  Standard  Riley  Underfeed 
stokers  (Fig.  92)  arranged  in  a  Duplex  setting.  The  furnace 
width  is  24  ft.  and  the  depth  about  17  ft.  6  inches.  The  total 
grate  area  under  each  boiler  is  417.8  square  feet.  The  stokers 
are  set  so  that  there  is  a  height  of  10  ft.  10  inches  from  the 
top  of  the  grate  to  the  front  header  and  6  ft.  from  the  top 
of  grate  to  rear  header.  When  the  boilers  are  operating  at 
normal  rating  at  275  pounds  working  pressure  and  275°  super- 
heat, the  rate  of  coal  feed  by  the  plungers  is  127  pounds  per 
retort  per  hour.  When  feeding  700  pounds  of  coal  per  retort 
per  hour  the  boilers  will  operate  at  about  500%  of  boiler  rating. 
Estimating  the  average  thickness  of  the  fuel  bed  as  two  feet 
there  would  be  in  the  furnace  at  all  times  about  twenty-three 
tons  of  green  coal  and  coal  in  process  of  combustion.  The 
equipment  is  designed  to  operate  between  500%  and  600%  of 
boiler  rating  for  short  periods. 


CLEVELAND  ELECTRIC  ILLUMINATING  COMPANY, 
Cleveland,  Ohio 

The  Lake  Shore  station  of  the  above  company  is  a  large 
central  station  furnishing  electricity  for  the  community  in  and 
around  Cleveland.  The  boiler-room  equipment  consists  of  class 
M-25  685  horse  power  Stirling  Boilers,  equipped  with  12  ft.  wide 
by  15  ft.  long  Green  Chain  Grate  stokers.  These  stokers  are 
of  the  combination  KLM  type  Green  design  for  handling  free 
burning  coal.  The  boilers  are  set  with  what  is  known  as  a 
rear  end  setting.  The  central  line  of  the  mud  drum  being 
15'  8"  above  the  floor  line  (Fig.  93).  This  gives  an  unusually 
large  combustion  space,  there  being  16  cubic  feet  furnace  volume 
per  square  foot  of  grate  surface. 

The  coal  generally  used  at  this  station  comes  from  South- 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS   197 


FIG.  92. — Setting  of  Buffalo  General  Electric  Stokers,  Showing  Duplex  Setting 

of  Riley  Stokers. 


198 


MECHANICAL  STOKERS 


.  I     16 "x?/  'Opening in  Side  Wall 

Opening  in  Side  Girder 


l<-3-6*--> 

Longitudinal   Section 
FIG.  93.— Setting  of  Green  Chain  Grate  Stokers  with  Stirling  Boilers,  Cleveland 
Electric  Illuminating  Company. 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    199 

eastern  Ohio,  which  is  free  burning  coal.    The  analysis  running 
about  as  follows: 

Moisture    2.90 

Volatile  Matter 31.4 

Fixed  Carbon 55.0 

Ash 13.6 

Sulphur  4.4 

B.T.U.  (dry)    12,350 

B.T.U.   (as  fired)    11,992 

The  stoker  equipment  was  designed  for  a  range  of  operation 
from  100%  to  300%  of  boiler  rating.  The  average  operating 
rating  being  about  175%. 

The  stoker  equipment  is  designed  to  give  from  70%  to  75% 
combined  boiler  and  stoker  efficiency  when  operating  at  this 
rating. 

DUQUESNE  LIGHT   COMPANY, 
Pittsburgh,  Pa. 

The  "  Coif  ax  Station"  of  the  above  company  located  at 
Springdale,  Pa.,  on  the  Allegheny  River  was  designed  for  an 
ultimate  capacity  of  300,000  K.W.  The  location  of  this  station 
makes  several  sources  of  coal  supply  available.  The  stoker 
equipment  consists  of  17  retorts,  Westinghouse  Underfeed 
stokers  applied  to  B.  &  "W.  Cross  Drum  boilers,  each  having 
heating  surface  of  20,706  square  feet.  The  ratio  of  furnace 
volume  to  boiler  rating  is  3.45  cubic  feet.  The  boilers  are  18 
tubes  high  by  15  tubes  wide  (Fig.  94).  The  stoker  equipment 
was  designed  for  burning  high  volatile  Pittsburgh  coals  of 
the  following  analysis: 

Fixed   Carbon 54.00 

Volatile    34.00 

Moisture    3.00 

Ash    9.00 

Sulphur 1.23 

B.T.U.   (as  fired)    13,500 

Double  roll  clinker  grinders  were  installed,  the  same  being 
cooled  by  a  continuous  water  spray.  To  eliminate  as  much  as 


200 


MECHANICAL  STOKERS 


possible  the  formation  of  clinkers,  air  boxes  were  designed  and 
installed  in  the  side  and  bridge  walls.  The  boiler  and  stoker 
efficiency  of  the  plant  under  regular  operating  conditions  were 


FIG.  94.— Setting  of  Westinghouse  Stokers  and  B.  &  W.  Boilers,  Coif  ax 
Station — Duquesne  Light  Company. 

designed  for  78%  at  100%  of  boiler  rating  and  65%  at  300% 
of  boiler  rating.  The  entire  stoker  equipment  was  designed  for 
250%  of  boiler  rating. 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS  201 

WEST  PENN  POWER  COMPANY, 
Pittsburgh,  Pa. 

The  design  of  the  new  plant  of  the  West  Penn  Power  Com- 
pany, on  the  Allegheny  river  above  Pittsburgh,  contemplated 
some  decidedly  novel  features  in  the  boiler  and  stoker  equip- 
ment. The  initial  installation  was  designed  for  six  boilers  of 


FIG.  95. — Stoker  and  Boiler  Installation,  West  Penn  Power  Company. 

the  cross-drum  vertical-header  type,  42  sections  wide,  16  tubes 
high,  20  ft.  long,  set  with  the  front  header  16  ft.  above  the 
floor,  each  boiler  being  rated  at  1,529  H.P.  and  equipped  with 
superheaters  designed  to  give  200  degrees  superheat.  Westing- 
house  Underfeed  stokers  are  installed  at  the  front  and  rear 
ends  of  the  boilers  (Fig.  95)  14  retorts  under  the  mud  drum, 
and  14  retorts  under  the  front  of  the  boiler.  The  operating 
conditions  being  a  maximum  of  300  pounds  gauge  pressure, 
200  degrees  superheat. 


202 


MECHANICAL  STOKERS 


The  boilers  are  set  in  two  rows  with  aisles  about  15  ft. 
between,  thus  giving  plenty  of  room  around  each  boiler  for 
proper  operating  facilities.  The  stoker  drives  are  so  divided 
that  there  are  7  or  14  retorts  driven  by  one  prime  mover,  and 
the  wind  box  dampers  are  arranged  to  control  separately  the 
air  for  units  for  three  or  four  retorts.  The  stokers  are  equipped 
with  clinker  grinders  for  continuously  removing  the  ash  and 
clinker.  Pittsburgh  coals  with  approximately  the  following 
analyses,  are  used: 


Coal  "A" 

Coal  "B" 

Coal"C" 

Fixed  carbon 

57  38 

49  56 

56  55 

Volatile  

34  81 

32  84 

32  80 

Ash  

7  81 

13  26 

10  10 

Moisture 

5  52 

0  94 

0  55 

Sulphur  
B.T.U.  fas  fired).  . 

1.50 
13.500 

1.20 
11.748 

0.79 
12.713 

The  boiler  equipment  is  designed  so  that  the  flue  gas  tem- 
peratures will  range  from  500  degrees  at  150  per  cent  rating 
to  700  degrees  at  300  per  cent  rating,  the  combined  efficiency 
ranging  from  75  per  cent  at  150  per  cent  rating,  to  65  per  cent 
at  350  per  cent  rating.  Each  stoker,  when  burning  fuel  as 
mentioned  above,  is  designed  to  develop  350  per  cent  of  boiler 
rating  continuous  with  the  clinker  grinder  in  operation,  and 
400  per  cent  of  boiler  rating  for  peaks  of  short  duration.  Under 
these  operating  conditions,  the  combustible  in  ash  is  not  to 
exceed  14  per  cent. 


AMERICAN  GAS   &  ELECTRIC  CO., 
Windsor,  W.  Va. 

The  plant  of  this  company,  located  in  the  coal  fields  of 
Pittsburgh,  is  one  of  the  largest  power  plant  developments. 
The  present  boiler-room  equipment,  either  installed  or  provided 
for,  consists  of  14  boilers  with  underfeed  stokers  similar  to  the 
equipment  mentioned  for  the  Union  Gas  &  Electric  Co.  The 
setting  of  the  stokers  is  shown  in  Fig.  96. 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS  203 

DETROIT  EDISON  COMPANY, 

Detroit,  Mich. 

The  first  installation  of  double  set  underfeed  stokers  under 
large  boilers  was  made  at  the  Delray  Plant  of  the  Detroit 


F»n  and  Motor  Drive  (f) 
Dampers  In  Fan  Outlet  (F) 


FIG.  96.— Stoker  and  Boiler  Setting  American  Gas  &  Electric  Co. 

Edison  Company.  This  consisted  of  two  13-retort  Taylor 
Stokers,  under  each  of  the  2365  horse  power  water  tube  boilers, 
as  shown  in  Fig.  97.  Roll  clinker  grinders  were  developed 
for  these  stokers  and  have  been  installed  on  the  eight  Taylor 
Stokered  boilers  of  this  type  at  Delray. 


204 


MECHANICAL  STOKERS 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS  205 

This  equipment  was  duplicated  for  the  ten  units  compris- 
ing the  boiler  and  stoker  equipment  at  the  Connors  Creek 
Station  of  this  Company. 

On  test  the  efficiencies  of  these  units  have  run  from  80% 
at  slightly  above  rating  to  approximately  77%  at  200%  of 
rating,  when  burning  coal  of  approximately  the  following 
analysis : 

Moisture    2.00 

Volatile  33.00 

Fixed  Carbon    / 61.00 

Ash 6.00 

B.  T.  U.  per  pound  (as  fired) 14,000 

The  Congress  Street,  Willis  Avenue  and  Farmer  Street 
Stations  have  similar  double  set  Taylor  Stoker  installations, 
but  with  a  smaller  number  of  retorts  per  unit.  The  Marysville 
Plant  will  have  twelve-retorts,  twenty-two  tuyeres,  double  set 
Taylor  Stokers  under  2365  horse  power  boilers.  This  equipment 
will  burn  sufficient  coal  of  14,000  B.T.U.  per  pound  as  fired  to 
develop  350%  of  boiler  rating.  The  efficiencies  will  range  from 
80%  at  150%  of  rating  to  75%  at  250%  of  rating. 

UNION  GAS    &  ELECTRIC  CO., 
Cincinnati,  Ohio. 

In  the  new  plant  of  this  company  there  are  installed  cross- 
drum  type  boilers  containing  approximately  12,625  sq.  ft.  of 
water  heating  surface,  with  superheaters  to  produce  250  degrees 
superheat.  Each  boiler  was  made  up  of  42  sections  each,  13 
tubes  high  and  20  ft.  long,  the  furnace  width  being  24  ft. 
inside  the  setting  walls.  Each  boiler  is  equipped  with  econo- 
mizers over  the  boiler  and  each  boiler,  with  its  economizer,  is 
designed  for  evaporating  100,000  pounds  of  water  per  hour 
continuously  from  100  degrees  to  steam  at  250  pounds  pressure, 
and  superheated  250  degrees.  The  entire  equipment  is  capable 
of  evaporating  120,000  of  water  under  the  same  conditions  for 
short  periods.  The  fuel-burning  equipment  is  designed  for 
burning  West  Virginia  coal  from  the  Kanawha  district,  con- 
taining approximately  12,500  B.T.U.  per  Ib.  as  fired.  The  setting 
of  these  stokers  is  shown  in  Fig.  96. 


206  MECHANICAL  STOKERS 

The  stoker  equipment  is  of  the  Westinghouse  &  Riley  under- 
fed type,  each  stoker  containing  14  retorts  placed  under  the 
rear  of  the  boiler  under  the  mud  drum.  The  stokers  consist 
of  double  dumping  grates  with  arrangements  for  admitting  air 
to  them.  The  fuel-burning  equipment  is  designed  for  com- 
bined efficiency  ranging  from  75  per  cent,  with  a  boiler  capacity 
of  35,000  Ibs.  of  water,  to  65  per  cent  with  a  capacity  of  100,000 
Ibs.  of  water.  Each  stoker  is  driven  independently  by  direct- 
current  motors  connected  by  silent  chain  drives  to  the  line 
shaft  of  the  stokers.  Instrument  boards  are  installed  to 
indicate  to  the  operators  the  exact  furnace  conditions. 


MERCHANTS  HEAT  &  LIGHT  CO., 
Indianapolis,  Ind. 

The  above  company  replaced  their  former  coal-burning 
equipment  with  Westinghouse  underfeed  stokers  and,  at  the 
same  time,  added  additional  800  horse  power  units  to  their 
plant,  the  entire  work  consisting  as  follows : 

Twelve  500  H.P.  Stirling  boilers  equipped  with  twelve  5- 
retort  Westinghouse  underfeed  stokers. 

Two  800  H.P.  vertical  boilers  equipped  with  two  9-retort 
Westinghouse  underfeed  stokers. 

Two  800  H.P.  Badenhausen  boilers  equipped  with  two  8- 
retort  Westinghouse  underfeed  stokers. 

In  the  new  settings,  the  lower  drum  of  the  boiler  has  been 
raised  considerably  so  that  doors  could  be  placed  in  the  bridge- 
wall. 

The  fuel-burning  equipment  was  laid  out  to  burn  Indiana 
screenings  of  the  following  analysis,  and  when  burning  this 
fuel  the  operating  performance  ranges  from  100%  boiler  rating 
to  300%  boiler  rating  for  short  durations: 

Fixed  Carbon  43.89 

Volatile  40.27 

Moisture    15.23 

Ash 15.84 

Sulphur  . 3.99 

B.T.U 12,053 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    207 


UNION  ELECTRIC  LIGHT   &  POWER  CO., 
St.  Louis,  Mo. 

The  boiler  plant  of  this  company  has  been  entirely  re- 
vamped and  a  change  made  in  the  type  of  fuel-burning  equip- 
ment formerly  used.  Careful  study  was  made  in  regard  to  the 
installation  of  stokers,  and  it  was  finally  decided  to  install 
underfeed  stokers  for  use  with  good  Illinois  coal  of  the  follow- 
ing analysis: 

Fixed  Carbon 48.9 

Volatile  27.3 

Ash    14.9 

Moisture    9.0 

B.T.U.  (as  fired)    11,112 

The  main  problem  at  the  start  was  that  of  designing  the 
equipment  to  eliminate,  as  much  as  possible,  trouble  due  to 
clinker  formation  on  the  side  walls.  After  the  equipment  was 
in  operation,  and  when  using  the  coal  that  was  originally 
contemplated,  very  little  difficulty  was  encountered  with 
clinkers. 

Performance  results  are  shown  in  Fig.  98,  when  a  grade 
of  coal  with  the  following  analysis  was  used: 

Fixed  Carbon    41.0 

Volatile 29.0 

Moisture   9.0 

Ash    21.7 

B.T.U.    (dry)     12,000 

With  this  coal  considerable  more  attention  was  required  to 
keep  the  fires  uniform  and  cleaned  properly  in  order  to  decrease 
clinker  trouble  to  a  minimum. 


COMMONWEALTH  EDISON  COMPANY, 
Chicago,  111. 

The  Northwest  Station  of  the  above  company  located  on 
the  Chicago  River  is  one  of  the  largest  central  stations  using 
chain  grate  type  of  stokers. 


208 


MECHANICAL  STOKERS 


The  first  section  of  this  plant  included  580  horse  power 
B.  &  W.  boilers  operated  at  a  pressure  of  250  Ibs.  with  super- 
heat of  125°.  The  stoker  equipment  consisted  of  B.  &  W. 
chain  grate  stokers,  twenty  of  which  have  150  sq.  ft.  of  grate 
surface  each  and  twenty,  136  sq.  ft.  grate  surface. 

The  last  boilers  installed  were  1220  horse  power  cross  drum 
B.  &  W.  type,  pressure  250  Ibs.  superheat  220°. 


0  140  150  160  170  IbO  ISO  200  2  0  220  230  240  250  260  2  0  280  290  300  310  320 


FIG.  98. — Performance  of  Underfeed  Stoker  when  Burning  Carterville  Illinois 

Coal. 

Illinois  coal  is  used  at  this  plant  averaging  about  10,000 
B.T.U.  as  fired. 


MINNEAPOLIS  GENERAL  ELECTRIC  COMPANY, 
Minneapolis,  Minn. 

In  re-designing  the  fuel-burning  equipment  of  the  plant 
of  the  Minneapolis  General  Electric  Company  (Fig.  99), 
there  were  installed  12  Westinghouse  underfeed  stokers  under 
twelve  600  H.P.  boilers.  A  recent  extension  to  this  contemplated 


EQUIPMENT  OF  MODERN  STEAM  POWER  STATIONS    209 


the  installation  of  five  14-retort  underfeed  stokers  under  five 
1300  H.P.  boilers.  On  account  of  the  coal  conditions  prevailing 
at  this  plant,  it  was  necessary  to  design  equipment  for  two 
grades  of  coal  of  the  following  proximate  analyses: 


Coal  "A" 

Coal  "B" 

Fixed  carbon  
Volatile 

56.48 
30  81 

43.49 
32  59 

Ash 

11  03 

20  44 

Moisture             

7  00 

10.00 

Sulphur 

1  70 

3  48 

B.T.U  

13,400  dry 

11,200  dry 

FIG.  99.— Underfeed  Stoker  Equipment  of  the  Minneapolis  General  Electric 

Company. 

Under  the  above  conditions,  the  operating  performance  of 
fuel  "A"  ranged  from  1800  to  3000  H.P.  continuous  and  4500 
H.P.  for  short  durations.  With  the  poorer  grade  of  coal,  the 
Maximum  capacity  was  reduced  to  3600  H.P.  for  short  duration. 


210  MECHANICAL  STOKERS 

DENVER  GAS   &  ELECTRIC  COMPANY 
Denver,  Colo. 

Recent  developments  in  the  West  have  brought  about  the 
installation  of  underfeed  stokers  for  burning  coals  found  in 
Denver  markets.  The  above  company 's  new  extensions  included 
the  installation  of  four  750  H.P.  boilers  and  four  nine  retort 
Westinghouse  stokers.  The  stoker  application  setting,  worked 
out  gives  sufficient  combustion  space  for  any  high  volatile  coals, 
including  lignite,  that  are  liable  to  be  used  at  this  plant.  The 
fuel-burning  equipment  is  designed  for  the  following  coals: 

Fixed  Carbon 39.00 

Volatile  35.85 

Moisture    19.70 

Ash    5.37 

Sulphur 0.42 

B.T.U.    (dry)     12,000 

When  using  the  above  fuel,  the  operating  performance 
ranges  from  140%  boiler  rating  to  200%  boiler  rating  for  short 
duration,  with  approximately  70%  combined  boiler  and  furnace 
efficiency. 


CHAPTER  VIII 
APPLICATION  OF  STOKERS 

DETERMINATION  OF  SIZE 

While  it  is  true  that  economy  in  fuel  burning  is  largely  the 
result  of  good  operation  it  is  necessary  that  the  apparatus  be 
properly  proportioned  to  the  work  to  be  done.  A  poorly 
proportioned  installation  if  skillfully  operated  may  give  as 
good  or  better  results  than  one  correctly  proportioned  and 
carelessly  operated,  but  this  does  not  relieve  the  engineer  of 
responsibility  for  correct  design  and  proportions. 

Under  best  conditions,  the  range  of  efficient  combustion 
rates  is  not  great  and  under  average  operating  conditions, 
this  range  is  even  smaller.  As  combustion  rates  are  increased 
above  the  most  efficient  point,  the  amount  of  excess  air  will 
remain  constant  or  may  decrease  slightly,  indicating  an  increase 
in  efficiency  but  this  is  more  than  offset  by  increased  ash  pit 
loss  and  the  possibility  of  incomplete  combustion.  Below  the 
point  of  best  efficiency  the  ash  pit  loss  should  decrease  but 
the  loss  due  to  excess  air  will  increase  more  than  enough  to 
offset  the  decreased  ash  pit  loss  and  the  net  efficiency  decreases. 
The  rate  of  this  decrease  depends  upon  the  skill  and  care  of 
the  operators  and  in  many  plants  is  so  rapid  that  the  range  of 
efficient  combustion  rates  is  very  small.  Fig.  100  shows  the 
range  for  best  conditions,  the  efficiency  curve  indicating  what 
actually  happens  in  many  plants. 

It  is  apparent  that  the  engineer  who  selects  the  stoker  and 
boiler  must  carefully  consider  the  plant  load  conditions  and 
select  the  correct  proportions  in  order  that  the  operators  may 
be  able  to  secure  the  best  results. 

If  the  plant  has  a  substantially  uniform  load  throughout 
the  twenty-four  hours,  the  best  results  so  far  as  the  boiler  is 

211 


212 


MECHANICAL  STOKERS 


concerned  will  be  secured  at  between  125%  and  175%  of 
rating.  Variations  within  this  range  will  depend  upon  local 
conditions  such  as  cost  of  real  estate,  price  and  quality  of  fuel 
and  necessity  of  providing  future  development. 

For  plants  carrying  a  uniform  load  for  from  eight  to  twelve 
hours  a  day  and  banked  the  balance  of  the  time,  the  fixed 
charges  per  unit  of  output  will  be  greater  and  it  is  necessary 
to  operate  at  higher  ratings  to  secure  the  lowest  operating 
cost.  Under  such  conditions,  the  most  economical  rating  will 
be  between  150  and  200%,  the  exact  figure  being  determined 
by  local  conditions. 


r 

W  -TA 

"~"»  fc^^^^^ta 

•o  (* 

f-,S 

3? 

^X> 

\ 

X 

% 

0  -TQ 

/^ 

/ 

s 

fc 

"-  08 

£ 

f# 

s 

-0 

o 

j_  ££ 

/ 

/ 

\ 

\ 

_£ 

£01 

( 

37 

N      / 

\ 

-o 

V 

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1 

r 

\ 

<->  60 

/ 

^\ 

)                    5 

0                   1C 

0                  1! 

JO                 ZC 

)0                 25 

0                30( 

Per  Cent  Boiler  Rating 
FIG.  100. — Typical  Capacity-efficiency  Curve. 

The  twenty-four  hour  variable  load  of  the  central  station 
plant  presents  additional  difficulties.  The  maximum  capacity 
must  be  ample  for  the  peak  load  and  since  the  maximum  peak 
occurs  only  a  few  times  each  year,  the  efficiency  at  which  this 
peak  can  be  carried  is  of  little  importance.  The  best  results 
will  be  secured  by  proportioning  the  apparatus  to  carry  the 
day  load  most  economically  and  providing  the  necessary  over- 
load capacity  to  meet  the  peaks.  If  the  day  load  can  be  carried 
at  between  150%  and  200%  rating  and  the  peaks  by  increas- 
ing to  250%  or  300%,  which  represents  the  average  condition, 
a  satisfactory  combination  can  be  secured.  If  a  greater  peak 
load  capacity  is  necessary,  it  is  advisable  to  install  stokers  of 


APPLICATION  OF  STOKERS  213 

sufficient  capacity  to  meet  this  demand  rather  than  to  increase 
the  size  and  number  of  boilers,  provided  the  peak  load  is  not 
more  than  twice  the  day  load.  In  general,  it  is  not  wise  to 
design  for  a  peak  load  on  any  unit  of  more  than  twice  the 
average  day  load  because  at  ratings  below  50%  of  the  maximum 
capacity,  the  efficiency  will  fall  rapidly  unless  the  best  of 
operation  can  be  depended  upon. 

Tlie  intermittent  load  of  some  industrial  plants  presents 
a  particularly  difficult  problem.  Sudden  and  unexpected  steam 
demands  must  be  met,  in  some  cases  without  a  large  drop  in 
steam  pressure,  and  .the  equipment  must  be  able  to  satisfy 
these  demands  regardless  of  economy.  The  selection  of  equip- 
ment must  be  such  that  the  maximum  demand  can  be  supplied 
and  the  flexibility  should  enable  the  periods  of  low  load  to  be 
carried  economically,  while  the  change  from  one  condition  to 
the  other  should  be  made  quickly  and  automatically.  Such  a 
combination  of  requirements  cannot  always  be  met  and  the 
economy  of  operation  will  necessarily  be  lower  than  in  a  plant 
having  a  better  load  condition. 

The  selection  of  boiler  equipment  should  never  be  made 
without  at  the  same  time  considering  the  stokers,  because  the 
boiler  proportions  are  determined  largely  by  the  size  of  stoker 
selected.  A  typical  example  will  make  this  clear:  Assume  a 
plant  being  designed  to  carry  a  steady  load  of  3000  B.H.P. 
burning  coal  having  13,500  B.T.U.  per  Ib.  on  underfeed  stokers, 
and  that  a  rating  of  about  150%  has  been  decided  upon  as  the 
most  economical  rating.  This  load  could  be  carried  on  four 
500  H.P.  units,  each  developing  150%  rating  and  the  plant 
should  contain  five  units,  allowing  one  spare.  The  stokers 
should  be  of  such  size  that  three  units  could  carry  the  load  in 
case  of  a  forced  shut  down  of  one  unit.  This  requires  200% 
rating  from  each  of  the  remaining  units  and  it  might  be  neces- 
sary to  carry  this  rating  for  twenty-four  hours.  The  stokers 
must  therefore  be  of  sufficient  size  to  operate  the  boilers  at 
200%  rating  for  twenty-four  hours.  Assuming  72%  combined 
efficiency,  the  coal  consumption  per  boiler  per  hour  at  200% 
rating  will  be 


214  MECHANICAL  STOKERS 

at  150%  rating  and  75%  efficiency,  the  coal  burned  per  boiler 
per  hour  will  be 

34.5X970.4 

13,500  X.  75  X 

A  stoker  must  be  selected  that  will  burn  3440  Ibs.  of  coal  per 
hour  for  twenty-four  hours,  but  the  most  important  requirement 
is  high  economy  when  burning  2483  Ibs.  per  hour.  For  the 
given  condition  a  combustion  rate  of  35  Ibs.  per  square  foot  per 
hour  should  give  the  best  results  and  the  stoker  size  will  be 

2,483  ..    . 

-  =  71.  sq.ft., 


at  200%  rating  this  stoker  would  have  a  combustion  rate  of 
3,440 


71 


48.5  Ibs.  per  square  foot  per  hour, 


which  is  well  within  the  reserve  capacity  of  the  type  of  stoker 
selected.  These  stokers  are  made  in  lengths  of  9  ft.  or  more 
and  it  is  therefore  unnecessary  to  make  the  stoker  more  than 

^  =  7.9  ft.  wide, 
y 

In  order  to  allow  for  the  variations  in  dimensions  employed 
by  different  manufacturers,  it  would  be  desirable  to  select  a 
boiler  having  a  furnace  width  of  9'  0"  or  the  nearest  standard 
width.  Standard  boiler  designs  for  units  of  500  H.P.  can  be 
secured  in  furnace  widths  up  to  14'  0"  and  it  is  obvious  that 
if  such  a  design  had  been  purchased  without  considering  the 
stoker  proportions,  a  serious  mistake  would  have  been  made. 

In  general,  it  may  be  said  that  any  given  size  or  type  of 
boiler  will  give  better  results  if  made  as  narrow  as  possible 
because  the  narrow  unit  gives  a  better  distribution  of  the  heat- 
ing surface  and  will  be  a  more  efficient  heat  absorber.  The 
stoker  should  be  proportioned  with  this  fact  in  mind  and 
where  several  combinations  of  width  and  length  are  available, 
the  narrow  long  one  should  be  selected  and  the  boiler  pro- 
portioned accordingly. 

In  order  that  the  best  stoker  size  may  be  selected  for  any 
given  condition,  it  is  necessary  that  the  best  combustion  rates 


APPLICATION  OF  STOKERS 


215 


for  the  various  grades  of  coal  be  known.    Figs.  101,  102  and 
103  give  these  values  for  several  typical  fuels. 

The  minimum  combustion  rate  recommended  for  continuous 
operation  represents  the  lower  limit  for  actual  operation 
although  good  test  results  can  be  secured  at  lower  rates.  This 
requires  careful  operation  and  constant  attention  such  as  fires 
receive  during  tests  or  in  regular  operation  in  a  few  excep- 
tionally well  managed  plants.  Under  average  operating  con- 
ditions, however,  the  efficiency  will  drop  off  rapidly  at  com- 
bustion rates  below  this  minimum  and  unless  it  is  absolutely 


Combustion 
Rate  per  Sq. 
Ft.  per  Hour 
Dry  Coal 

EASTERN  COAL 

FC.             73 

vol.          n 

Ash              6 
BT.U.(Dry)     14300 

PITTSBURGH  COAL 

EC.              57 
Vol.             30 
Ash               7 
S                Z 

Moisture       4 
B.T.U.(Dry)      13500 

ILLINOIS  COAL 

FC.              48 
Vol.              30 
Ash            12 

Moisture      10 
BIlWDry)     12200 

IOWA  COAL 

FC.              33 
Vol.             Zl 

%"       ? 

Moisture       15 
&T.U.(Dry)     10400 

LIGNITE 

FC.         34 
Vol.         33 
Ash         10 
S>.             1 

Moisture   23 
B.T.U.(Dru)  11500 

Mmimunrfbr 
Continuous 
Operation 

20-25 

25-28 

25-28 

25-28 

25-28 

Recommended 
for  Continuous 
Operation 

30-38 

32-40 

30-38 

28  -35 

30-35 

Maximum  for 
Continuous 
Operation 

40-45 

40-45 

38-42 

35-42 

38-45 

Recommended 
for  3  to  4  Hr. 
Peaks 

50-60 

50-60 

45-50 

42-45 

45-50 

Maximum  for 
3  to  4  Hn 
Peaks 

60  -65 

60-65 

50-55 

45-47 

50-55 

Maximum 
for  One  Hr. 
Peaks 

10 

70 

60 

50 

60 

FIG.  101. — Combustion  Rates  Recommended    for  Various  Fuel    Conditions 
for  Forced  Draft  Underfeed  Stokers. 

necessary,  provision  should  not  be  made  for  such  operation. 
Some  industrial  plants  having  an  intermittent  load  may  be 
required  to  operate  below  this  minimum  but  cannot  reduce  the 
number  of  units  in  service  on  account  of  the  periods  of  heavy 
load  which  alternate  with  the  low  load  periods,  and  such 
plants  are  at  a  definite  disadvantage  so  far  as  economy  is 
concerned. 

The  continuous  combustion  rates  recommended  for  best 
results  apply  especially  to  plants  having  steady  loads.  It  is 
usually  necessary,  however,  to  effect  a  compromise  between 
this  combustion  rate  and  the  maximum  continuous  rate.  In 
the  case  cited  above  it  is  desired  to  carry  a  given  load  on  four 


216 


MECHANICAL  STOKERS 


boilers  which  in  an  emergency  can  be  carried  on  three  of  the 
units.  The  most  efficient  combustion  rate  was  thirty-five  Ibs. 
per  sq.  ft.  of  grate  surface  per  hour  and  four  units  at  this 


Combustion 
Rate  per  Sq. 
Ft.  per  Hour 
Dry  Coal 

EASTERN  UAL 

F.C.             73 

vol.          n 

Ist1     ? 

Moisture       4 
B.T.U(Dry)    14300 

PITTSBURGH  COAL 

FC.               5T 
Vol.              30 

f        I 

Moisture      4 

B.T.U.(OryV     13500 

ILLINOIS  COAL 

F.C.               48 
Vol.              30 

gi.       3 

Moisture       10 
B.T.U.(Dru)     12200 

IOWA  COAL 

F.C.               33 
Vol.               27 
£,            « 

Moisture       15 
BJ.U.(Dry)     10400 

LIGNITE 

Vol'.         33 
Ash         10 
&              1 

Moisture    ZT> 
B.T.U.(Ory)  11500 

Minimum  for 
Continuous 
Operation 

15-  ia 

18  -20 

18-20 

18-20 

18-20 

Recommended 
for  Continuous 
Operation 

20  -25 

23-26 

23-26 

20-23 

72-26 

Maximum  for 
Continuous 
Operation 

25  -t8 

"50-35 

•50-32 

25-27 

26-32 

Recommended 
for3to4Hr. 
Peaks 

•50-35 

35-40 

32-35 

27-30 

32-35 

Maximum  for 
3to4Hr. 
Peaks 

1)5-40 

40-42 

35-40 

30 

35 

Maximum 
for  One  Hr. 
Peaks 

40 

42 

40 

30 

35 

FIG.  102. — Combustion  Rates  Recommended  for  Various   Fuel   Conditions 
for  Natural  Draft,  Overfeed  Stokers. 


Combustion 
Rate  perSq. 
Ft.  per  Hour 
Dry  Coal 

EASTERN  COAL 

vol.           n 

Ash              6 
S  .                1 
Moisture      4 
BJ.U(Dru)     14500 

PITTSBURGH  COAL 

Vol.              30 

Alh 

Moisture      4 
B.T.U.(Ory)      13500 

ILLINOIS  COAL 

F.C.              48 
Vol.              30 
Ash             12 

Moisture       10 
B.T.U.(Ory)     12200 

IOWA  COAL 

F.C.              33 
Vol.              87 
Ash             25 
S                 4 
Moisture       15 
B.T.U.(Ory)     10400 

LI6NJTE 

F.C.         34 
Vol.         33 
Ash          10 

Moisture   23 
B.T.U.(Dr«)  H500 

Minimum  for 
Continuous 
Operation 

20-22 

ZO-22 

20-22 

20-22 

Recommended 
for  Continuous 
Operation 

23-26 

23-26 

22-25 

25-30 

Maximum  for 
Continuous 
Operation 

30-33 

32-35 

25-30 

35-40 

Recommended 
for3to4Hr. 
Peaks 

35-40 

40-45 

30-35 

42-45 

Maximum  for 
3to4Hr. 
Peaks 

40 

45 

35 

45 

[Maximum 
for  One  Hr. 
Peaks 

40 

45 

35 

45 

FIG.  103. — Combustion  Rates  Recommended   for  Various   Fuel  Conditions 
for  Natural  Draft,  Chain  Grates  and  Traveling  Grate  Stokers. 

rate  would  carry  the  load,  while  with  three  in  service,  the 
combustion  rate  would  increase  to  48.5  Ibs.  If  45  Ibs.  was  the 
maximum  for  continuous  operation  it  would  be  necessary  to 
increase  the  grate  area  to  76.5  sq.  ft.  thereby  reducing  the 


APPLICATION  OF  STOKERS  217 

combustion  rate  to  32.5  Ibs.  when  four  units  were  in  service. 
This  condition  does  not  enter  into  the  design  of  a  large  plant 
but  is  important  in  installations  of  three,  four  or  five  boilers. 

In  the  selection  of  new  apparatus,  it  is  possible  to  propor- 
tion both  boilers  and  stokers  for  best  results  but  in  the  case 
of  an  old  installation,  this  cannot  always  be  done.  Many 
boilers  which  are  still  in  condition  to  give  good  service  for 
years  and  therefore  cannot  be  discarded,  are  not  proportioned 
for  stoker  firing.  The  hand  fired  grate  is  limited  to  a  length 
of  about  seven  feet  and  it  has  been  common  practice  in  the 
past  to  make  the  boilers  wide  enough  to  permit  of  the  installa- 
tion of  the  desired  grate  area,  based  on  a  length  of  six  or 
seven  feet.  These  boilers  are  wider  than  would  be  selected 
today  for  stoker  firing,  but  it  is  often  necessary  to  equip  them 
with  stokers.  In  such  cases  it  is  important  that  the  stoker  be 
proportioned  to  suit  the  boiler  furnace  if  best  results  are  to 
be  secured.  The  stoker  should  be  made  as  wide  as  the  furnace 
if  possible  and  long  enough  to  give  the  desired  grate  area. 
Some  types  of  stokers  are  made  in  a  variety  of  lengths  and 
widths  and  are  therefore  more  easily  applied  to  these  special 
cases  than  those  designs  which  vary  only  in  width. 

The  * '  grate  area  "  of  a  stoker  is  a  rather  indefinite  unit  and 
depends  upon  the  type  of  stoker  and  the  methods  of  determina- 
tion employed  by  different  manufacturers  but  it  makes  little 
difference  how  this  unit  is  determined  because  it  does  not 
affect  the  fuel  burning  capacity  of  a  given  stoker.  If  a  manu- 
facturer chooses  to  designate  dump  grates,  coking  plates,  or 
dead  plates  as  "grate  area"  thereby  apparently  increasing  the 
size  of  the  stoker,  he  must  decrease  the  allowable  combustion 
rate  per  sq.  ft,  while  on  the  other  hand,  if  these  be  eliminated 
from  the  calculations,  the  unit  combustion  rate  will  be  cor- 
respondingly increased.  It  is  desirable,  however,  for  purposes 
of  comparison  that  the  same  standard  of  measurement  be 
applied  to  stokers  of  the  same  type  in  order  that  their  com- 
parative fuel  burning  capacities  can  be  compared. 

Overfeed  stokers  of  the  front  feed  type  usually  figure  the 
area  to  be  the  product  of  the  stoker  width  multiplied  by  the 
length  measured  along  the  line  of  fuel  travel  from  the  point 
where  the  fuel  enters  the  furnace  to  the  bridgewall.  In  some 


218 


MECHANICAL  &TOKERS 


cases,  the  dump  grates  are  not  included  and  in  others  they  are 
given  partial  values.  It  is,  therefore,  desirable  with  stokers 
of  this  type  to  determine  exactly  how  the  grate  area  has  been 
determined,  especially  when  a  comparison  is  to  be  made  with 
some  other  type  of  stoker. 

Side  feed  stokers  are  calculated  on  the  basis  of  the  dis- 
tance from  the  inside  of  front  wall  to  face  of  bridgewall,  mul- 
tiplied by  the  length  of  actual  grate  area  measured  along  the 


Each  ring  of  this  arch  must  be 

thoroughly  bonded  to  the  adjacent  rings 


FIG.  104. — Dimensions  of  Westinghouse  Roney  Stoker. 

grates.    For  convenience,  in  making  calculations,  the  projected 
area  can  be  multiplied  by  1.4. 

Multiple  retort  underfeed  stokers  have  generally  employed 
the  retort  as  a  unit  but  this  is  not  a  satisfactory  unit  of  grate 
area  because  no  two  manufacturers  have  adopted  the  same 
retort  dimensions  and  the  situation  is  further  complicated 
because  the  same  manufacturer  has  more  than  one  retort  size. 
It  is  now  generally  accepted  that  the  area  in  square  feet  should 
be  specified  and  this  is  determined  by  multiplying  the  actual 
width  by  the  horizontal  distance  from  the  inside  of  front  wall 
to  the  face  of  the  bridgewall.  This  includes  dump  grates  or 


APPLICATION  OF  STOKERS 


219 


other  ash  disposal  devices  but  does  not  take  into  account  the 
angle  of  the  retorts. 

Center  feed  underfeed  stokers  are  figured  on  the  basis  of 
projected  area  enclosed  by  the  four  furnace  walls. 

Traveling  grates  are  based  on  projected  area  enclosed  by 
the  side  walls,  feed  gate,  and  water  box.  Forced  draft  stokers 
of  this  type  do  not  employ  the  water  box  and  the  length  should 
be  measured  from  the  inside  of  the  feed  regulating  gate  to  the 
rear  of  the  last  wind  compartment. 

In  the  determination  of  correct  stoker  size  for  a  given  set 
of  conditions,  the  following  procedure  will  be  found  con- 
venient— 

Assume  the  following  conditions: 


Boiler  size,  500  H.P. 

Continuous  rating  desired 150% 

Maximum  rating  for  four  hours 200% 

Maximum  rating  for  one  hour 275% 

Coal— Volatile 32% 

Fixed  Carbon 54% 

Moisture 5% 

Ash 9% 

Sulphur 1% 

B.T.U.  dry 13,500 

Efficiency  at 150%        75% 

Efficiency  at 200%        72% 

Efficiency  at 275%        65% 


The  calculations  can  be  arranged  as  follows: 


Per  Cent 
Rating 

Horse  power 
Developed 

Efficiency 

Coal 
per  Horse  power 

Total  Coal 
per  Hour 

150 

750 

75 

3.31 

2483 

200 

1000 

72 

3.44 

3440 

275 

1375 

65 

3.82 

5253 

For  convenience  in  determing  the  pounds  of  coal  required 
per  boiler  H.P.,  a  diagram  such  as  Fig.  105  may  be  used  in 


220 


MECHANICAL  STOKERS 


the  following  manner:  locate  the  intersection  of  the  vertical 
line  of  B.T.U.  in  the  coal  with  the  diagonal  line  representing 
the  combined  efficiency  then  project  horizontally  from  this 
point  to  the  heavy  curved  line.  From  this  second  intersection, 
project  vertically  to  the  scale  representing  pounds  of  coal  per 
boiler  H.P.  The  same  diagram  may  be  used  to  determine  the 
equivalent  evaporation  per  pound  of  coal  by  locating  the  inter- 
section of  the  vertical  line  of  B.T.U.  in  the  coal  with  the 


Pounds  of  fuel  or  Combustible  Per  B.H.P  Per  flout 


4     35 


of 
& 


i.oVv 


ti& 


&' 


/  / 


<m 


Ui  Per 


of  fuel  ar  Cotbbustibk. 


FIG.  105.— Efficiency  Chart,  Giving  Pounds  of  Water  from  and  at  212°  F. 
per  Pound  of  Fuel  of  Specified  Heat  Values. 

diagonal  line  of  efficiency  and  projecting  horizontally  from  this 
intersection  to  the  scale  representing  equivalent  evaporation. 
The  stoker  selected  must  be  able  to  burn  coal  at  the  rate 
of  5253  Ibs.  per  hour  for  a  period  of  one  hour  and  should  be 
operating  close  to  its  most  efficient  combustion  rate  when  burn- 
ing 2483  Ibs.  per  hour  and  the  ability  of  a  stoker  to  meet  this 
condition  must  be  considered  in  selecting  the  type  best  suited. 
All  stokers  under  consideration  must  first  be  proportioned  to 
meet  the  maximum  demand  and  of  the  stokers  thus  proper- 


APPLICATION  OF  STOKERS 


221 


tioned  some  will  operate  more  efficiently  than  others  at  the 
average  rating.  This  often  has  a  greater  influence  on  the  type 
selected  than  any  other  factor.  When  the  type  has  been  selected, 
it  may  be  found  that  the  standard  size  built  by  one  manu- 
facturer meets  the  conditions  best,  and  this  fact  must  be  con- 
sidered in  comparing  details  of  stokers  of  the  type  which  has 
been  selected. 

When  the  weight  of  coal  to  be  burned  has  been  determined, 
it  is  then  necessary  to  select  the  best  rate  of  combustion  for 
the  stoker  under  consideration.  Figs.  101,  102  and  103  give 


(Furnace  Depth  ma;  be 
Decreased  to  the  above 
when  Limited  bj  Boiler 
Betting. 

Angles  Set  Flush  with  Side 
all  for  Securing  Door  and 


FIG.  106. — Dimensions  of  Riley  Underfeed  Stoker. 

the  combustion  rates  recommended  for  the  different  types  of 
stokers  and  grades  of  fuel.  Under  test  conditions,  lower  rates 
than  those  given  can  be  maintained  and  higher  rates  than  the 
maximum  values  can  easily  be  secured.  The  same  can  be  said 
of  many  well  operated  plants  but  these  figures  will  apply  to 
average  conditions. 

With  the  above  information  at  hand,  the  grate  area  re- 
quired for  a  given  condition  is  readily  determined.  The  next 
step  is  to  choose  the  standard  size  of  the  stoker  selected  which 
most  nearly  meets  the  requirements  and  it  is  therefore  neces- 
sary to  know  the  units  in  which  stokers  are  built. 


222  MECHANICAL  STOKERS 

SIZES  AND  DIMENSIONS  OF  THE  MURPHY  FURNACE 


Furnace 
Number 

Approx. 
Boiler  H.P. 

Grate 
Surface 
Projected 

Grate 
Surface 
Effective 

Width 
Furnace 

Depth 
Furnace 

1 

40 

9 

11.91 

3' 

3' 

2 

50 

12 

15.88 

3' 

4' 

3 

65 

14 

18.66 

3'  6" 

4' 

4 

72 

16 

23.31 

4' 

4' 

5 

80 

18 

26.37 

4'  6" 

4' 

6 

90 

20 

28.69 

4' 

5' 

7 

90 

20 

29.44 

5' 

4' 

8 

100 

22^ 

32.46 

4'  6" 

5' 

9 

125 

25 

36.23 

5' 

5' 

10 

135 

27 

38.55 

4'  6" 

6' 

11 

135 

27| 

40.00 

5'  6" 

5' 

12 

150 

30 

43.03 

5' 

6' 

13 

150 

30 

43.78 

6' 

5' 

14 

170 

33 

47.50 

5'  6" 

6' 

15 

190 

35 

49.82 

5' 

r 

16 

200 

36 

51.99 

6' 

6' 

17 

215 

38£ 

55.00 

5'  6" 

T 

18 

220 

39 

56.46 

6'  6" 

6' 

19 

230 

40 

58.12 

8' 

5' 

20 

250 

42 

60.06 

r 

6' 

21 

250 

42 

60.20 

6' 

7' 

22 

275 

45* 

65.37 

6'  6" 

r 

23 

290 

48 

68.41 

6' 

8' 

24 

290 

48 

69.02 

8' 

6' 

25 

300 

49 

69.55 

7' 

r 

26 

320 

52 

74.29 

6'  6" 

8' 

27 

325 

52* 

74.73 

7'  6" 

r 

28 

350 

56 

79.03 

7' 

8' 

29 

350 

56 

79.92 

8' 

1' 

30 

375 

60 

84.92 

T  6" 

8' 

31 

400 

63 

90.28 

9' 

r 

32 

400 

64 

90.81 

8' 

8' 

33 

450 

72 

102  .  60 

9' 

8' 

34 

500 

80 

114.38 

10' 

8' 

35 

550 

88 

126.16 

11' 

8' 

36 

600 

96 

137.94 

12' 

8' 

APPLICATION  OF  STOKERS 


223 


Overfeed  stokers  of  the  Murphy  and  Detroit  type  are  made 
in  width  of  from  3  ft.  to  8  ft.  varying  by  6  in.  and  in  depth 
from  3  ft.  to  8  ft.  varying  by  12  in.  When  the  furnace  width 
exceeds  12  ft.  two  stokers  are  installed  under  one  boiler. 

Underfeed  stokers  of  the  multiple  retort  type  vary  the  num- 
ber of  retorts  which  are  made  in  two  or  more  sizes.  Any 
desired  number  of  retorts  can  be  combined  into  one  stoker. 


STANDARD  SIZES — MULTIPLE  RETORT  STOKERS 
RILEY 


Type 

Length 

Retort 
Width 

Grate  Area,  Square  Feet 

Short 

8'    8" 

19" 

Number  of  retorts  X  13  72 

Standard  
Ex.  long  (short)  
Ex.  lone.  . 

9'    2" 
10'    8f" 
11'    22" 

19" 

19" 
19" 

Number  of  retorts  X  14  .  51 
Number  of  retorts  X  16  .  93 
Number  of  retorts  X  17  .  78 

Standard,  17  tuyere 

Long,  22  tuyere 

17  tuyere  (with  grinder) 
22  tuyere  (with  grinder) 


9'  0" 
10'  10" 

9'  6£' 
11'  4V 


TAYLOR 

20|" 
20!" 
20!" 
20!" 


Number  of  retorts  X 15 . 56+2 . 62 
Number  of  retorts  X 18 . 73  +3 . 16 
Number  of  retorts  X 16 . 50 + 2 . 78 
Number  of  retorts  X 19 . 67 +4 . 78 


WESTINGHOUSE 


14  tuyere  (double  dump) 
17  tuyere  (single  dump) 
17  tuyere  (double  dump) 
21  tuyere  (double  dump) 

8'  H" 

9'      !" 
10'      f" 
11'   7i" 

21" 
21" 
21" 
21" 

Number  of  retorts  X  14  .  18  +  0.8 
Number  of  retorts  X  15  .  86  +0  .  95 
Number  of  retorts  X  17  .  61  +  1  .  05 
Number  of  retorts  X  20  .  3  +1.21 

Center  retort  underfeed  stokers  are  made  in  single  units 
varying  in  width  from  5  ft.  to  14  ft.  varying  by  6  inches  and 
in  length  from  four  feet  to  ten  feet  varying  by  one  foot. 
Where  the  furnace  width  exceeds  fourteen  feet,  two  stokers 
are  installed  without  a  center  wall  between  the  units. 

Traveling  grates  are  made  in  width  from  four  feet  to 
fifteen  feet,  varying  by  six  inches  and  in  length  from  eight 


224 


MECHANICAL  STOKERS 


feet  to  seventeen  feet,  varying  by  one  foot.  Where  the  furnace 
width  exceeds  fifteen  feet,  two  stokers  are  installed  with  a 
center  wall  between  the  units. 


FIG.  107. — Dimensions  of  Westinghouse  Underfeed  Stoker. 


'Sfoker  Froni- 


-Stoker  Front 


L ^ 

Special  Standard 

FIG.  108. — Dimensions  of  Standard  Coal  Hoppers. 


APPLICATION  OF  STOKERS 


225 


CAPACITY  OF  COAL  HOPPERS  BASED  ON  COAL  AT  50  POUNDS  PER  CUBIC  FOOT 


XT           T>      J- 

IXri/^lfVi     /Tnoislf^ 

STANDARD  HOPPER 

SPECIAL  HOPPER 

No.  Retorts 

Wiatn  ^inside; 

Capacity,  Pounds 

Capacity  Pounds, 

2 

2'  6" 

281 

735 

3 

4'  3" 

478 

1250 

4 

6'  0" 

675 

1765 

5 

7'  9" 

872 

2280 

6 

9'  6" 

1069 

2795 

7 

11'  3" 

1266 

3310 

8 

13'  0" 

1463 

3825 

9 

14'  9" 

1660 

4340 

10 

16'  6" 

1857 

4855 

11 

18'  3" 

2054 

5370 

12 

20'  0" 

2251 

5885 

13 

21'  9" 

2448 

6400 

14 

23'  6" 

2645 

6915 

15 

25'  3" 

2842 

7430 

CHAPTER  IX 

INSTALLATION  OF  STOKERS— SPECIFICATIONS— CON- 
TRACTS— GUARANTEES— BOILER  ROOM  LOG 

There  has  been  some  improvements  in  recent  years  in  the 
matter  of  boiler  and  stoker  combinations.  Boilers  have  been 
raised,  stokers  have  been  set  differently;  more  attention  has 
been  given  to  the  breeching  design  and  higher  stacks  are  being 
used.  Still  there  has  never  been  anything  like  the  thought 
on  this  subject  that  there  should  be  and  the  standardization 
of  the  practice  that  is  necessary. 

There  is  no  reason  for  doing  exactly  the  same  things  that 
were  done  years  ago,  still  installations  are  going  on  nowdays 
the  same  as  they  did  fifteen  years  ago.  It  is  also  not  necessary 
to  present  exactly  the  same  standard.  When  changes  are  made, 
however,  there  is  need  of  an  analysis  of  the  installation  and 
the  information  translated  into  some  definite  grasp  of  the 
subject,  into  some  real  data  that  can  be  distributed  with  con- 
fidence. 

Whenever  a  stoker  is  installed  in  combination  with  a  boiler, 
there  are  many  things  that  must  be  considered  and  decided 
jointly  by  the  purchaser,  the  boiler  and  stoker  manufacturer. 
A  definite  conclusion  must  be  reached  on  the  following: 

1.  Height  of  the  boiler  header  above  the  floor  line — or  height  of  boiler 

setting. 

2.  Setting  of  the  stoker. 

3.  Combustion  space  necessary  for  the  coal  to  be  used. 

4.  Design  and  location  of  the  breeching. 

5.  Size  and  location  of  the  stack. 

6.  Area  of  gas  passages  through  the  boiler. 

7.  Area  of  damper  openings. 

8.  Facilities  for  cleaning  soot  off  boiler  baffles  and  boiler  tubes. 

9.  Grade  of  firebrick  to  be  used  in  the  furnace  construction. 

226 


INSTALLATION  OF  STOKERS  227 

10.  Size  of  walls  of  boiler  and  furnace  setting. 

11.  Size  and  grade  of  firebrick  for  arches. 

12.  Method  used  to  control  dampers. 

13.  Construction  of  ash  pit  and  facilities  for  disposing  of  the  ash. 

14.  Method  of  conveying  coal  to  the  stoker  hoppers. 

15.  Locations  of  fans,  etc. 

16.  Design  and  location  of  air  ducts. 


FIG.  109.— Typical  Underfeed  Stoker  Application. 


228  MECHANICAL  STOKERS 

If  the  purchaser  failed  to  install  the  height  of  stack  that 
was  necessary,  he  would  be  the  one  that  was  responsible  for 
the  failure.  If  the  stoker  manufacturer  did  not  take  a  firm 
stand  and  hold  out  for  the  correct  setting  of  the  stoker,  he 
would  be  responsible  for  the  failure.  If  the  boiler  manufac- 
turer failed  to  provide  the  correct  areas  for  the  passage  of  the 
furnace  gases  and  arrange  to  set  the  boiler  at  a  height  required 
for  the  particular  stoker  selected,  he  would  be  responsible 
for  the  failure. 

Of  course,  some  of  the  items  mentioned  are  more  important 
than  others.  Slighting  some  of  them  would  not  ruin  an  installa- 
tion entirely,  while  slighting  others  would  make  an  installation 
absolutely  a  failure. 

The  question  arises  as  to  how  those  interested  are  to  know 
to  what  degree  installations  will  be  successful.  One  can  hardly 
plan  an  installation  for  the  future  unless  he  has  some  concep- 
tion of  past  installations.  One  must  be  capable  of  recognizing 
the  strong  points  of  installations  and  ascertaining  the  weak 
ones.  Great  care  must  be  taken  to  see  that  the  weak  ones 
are  not  unduly  attributed  to  other  causes  than  the  real  ones. 
Names  sometimes  mislead.  Very  often  when  discussing  a  satis- 
factory installation  the  first  question  asked  is — What  is  the 
name  of  the  boiler  and  stoker?  In  most  cases  this  has  very 
little  to  do  with  the  results  obtained.  It  is  true  that  some 
stokers  adapt  themselves  more  easily  than  others  to  particular 
boilers;  and  it  may  be  true  that  some  boilers  are  more  easily 
set  than  others,  but  one  cannot  come  to  any  definite  con- 
clusion as  to  the  cause  for  a  satisfactory  or  unsatisfactory 
boiler  and  stoker  combination  unless  he  knows,  and  has  de- 
termined by  careful  analysis,  the  cause. 

It  is  not  intended  that  one  should  not  be  exact  and  resource- 
ful in  the  handling  of  various  materials  that  go  to  make  up 
a  good  boiler  and  stoker  combination.  If  it  is  not  necessary 
to  raise  boilers,  use  higher  stacks,  etc.,  the  reason  why  these 
things  are  not  necessary  should  be  known.  It  is  right,  how- 
ever, to  assert  what  we  actually  know  in  the  planning  of  new 
work. 

Installations  are  always  being  made  that  do  not  follow  the 
principles  that  are  commonly  known  to  be  right.  One  fre- 


INSTALLATION  OF  STOKERS  229 

quently  hears  that  boilers  cannot  be  set  higher  because  the 
contract  for  the  masonry  work  has  been  included  with  the 
boilers;  or,  that  the  contract  has  already  been  let  and  the 
purchaser  will  not  pay  the  extra  cost  of  setting  the  boilers 
the  way  they  should  be  set  with  the  particular  stoker  selected. 
Again,  we  frequently  hear  that  the  boilers  cannot  be  set  right 
because  the  architect  has  previously  provided  so  much  room 
for  the  boiler  and  stoker  and  they  must  go  within  the  limit 
provided,  whether  a  good  combination  or  a  bad  one  is  obtained. 
Plans  continue  to  be  made  in  the  wrong  way  and  unsatisfactory 
installations  result. 

There  is  another  phase  of  this  problem  that  seems  to 
interfere  with  obtaining  good  installations ;  that  is,  the  cost  of 
the  installation.  Of  course,  boilers  being  raised,  stokers 
extended,  higher  stacks,  etc.,  cost  more  money.  The  boiler  and 
stoker  manufacturers  must  take  a  sufficiently  firm  stand  for  the 
combinations  that  are  known  to  be  right  and  be  sure  that 
money  is  expended  to  make  them  right. 

A  case  is  recalled  where  the  contract  had  been  closed  for 
boilers,  stokers  and  the  brickwork.  The  boilers  were  to  be 
set  a  certain  height  and  the  contract  was  let  with  the  masonry 
figure  on  this  type  of  setting.  The  setting  did  not  agree  with 
what  the  stoker  manufacture  thought  was  right  and  he  took 
a  firm  stand  that  the  boiler  should  be  raised.  If  the  purchaser, 
the  boiler  and  stoker  manufacturers,  in  this  case,  had  cooperated 
and  definitely  decided  the  points  mentioned  in  the  forepart  of 
this  Chapter  enough  money  would  have  been  expended  to 
obtain  the  right  kind  of  a  combination. 

It  does  not  seem  possible  that  the  plans  and  methods  adopted 
in  one  section  of  the  United  States  could  be  followed  with 
the  same  degree  of  exactness  and  success  in  another,  quite 
remote,  or  any  other  place  where  coal  and  installation  con- 
ditions are  different.  It  would  seem  that  each  community 
where  coal  conditions  are  about  the  same  should  obtain  their 
own  data.  The  time  has  not  come  when  a  combination  can 
be  standardized  in  the  west  and  be  adapted  to  the  conditions 
in. the  east,  or  vice  versa,  with  success. 

Conditions  are  daily  encountered  that  are  prejudical  to 
good  furnace  performance.  One  plant  that  was  investigated 


230  MECHANICAL  STOKERS 

will  present  a  typical  example  of  conditions  to  be  found  in 
many  other  plants.  This  plant  operated  twenty-four  hours 
a  day  and  had  six  water  tube  boilers  installed,  each  of  250 
horse  power  rated  capacity.  The  boilers  were  served  by  one 
stack,  9  ft.  in  diameter  by  150  ft.  high.  There  was  one  long 
breeching  connecting  all  the  boilers.  A  short  breeching  con- 
nected the  main  breeching  to  the  stack.  The  complaints  were 
excessive  labor  and  great  difficulty  in  maintaining  the  steam 
pressure  necessary  to  operate  the  plant.  Considerable  money 
was  spent  in  sending  engineers  to  the  plant  with  instructions 
to  assist  in  every  possible  way  to  better  the  operating  con- 
ditions. It  was  immediately  found  that  the  draft  available 
in  each  furnace  was  low  and  insufficient  to  burn  the  coal 
required.  Arrangements  were  made  to  carefully  analyze  this 
draft  condition  and  find  the  cause  of  the  troubles.  There  was 
an  available  draft  in  the  stack  of  about  .90",  and  in  the  short 
breeching,  about  .83".  At  points  in  the  main  breechings,  how- 
ever, only  a  few  feet  away,  the  draft  dropped  to  about  .53", 
indicating  a  loss  of  .3"  draft  in  the  right  angle  turn  to  the 
main  breeching.  The  investigating  engineers  reported  the 
boilers  dirty  and  on  several  occasions  3/16"  scale  was  removed 
from  the  tubes.  It  was  also  found  that  the  boilers  were  only 
cleaned  every  six  months  and  the  soot  blown  from  the  tubes 
every  week.  There  was  a  heater  installed  and  owing  to  the 
piping  construction  it  could  only  be  cleaned  once  a  year.  An 
inspection  of  the  blow-off  valves  of  the  boiler  proved  that  they 
all  leaked. 

In  this  particular  case,  the  purchaser  was  at  fault  in  not 
operating  the  plant  properly  and  maintaining  the  equipment 
in  good  condition.  "Whoever  put  the  breeching  in  was  respon- 
sible for  its  faulty  design.  The  stoker  manufacturer  was  at 
fault  if  he  knew  of  the  breeching  design;  he  should  have 
cautioned  the  purchaser  regarding  draft  losses. 

A  peculiar  situation  arose  in  analyzing  the  conditions  of 
this  plant.  The  owner's  operating  engineer  reported  the  boiler 
free  from  scale.  The  investigating  engineer  reported  the 
boilers  badly  scaled.  It  was  finally  necessary  to  have  the 
manager  of  the  company  personally  inspect  the  boiler  on  which 
reports  were  being  made.  In  his  personal  investigations  small 


INSTALLATION  OF  STOKERS  231 

spots  of  scale  in  the  tubes  were  noted  which  the  turbine  had 
skipped.  The  operating  engineer  claimed  that  this  was  a 
trivial  matter.  The  manager,  however,  insisted  on  having  all 
of  the  scale  removed.  This  was  done  and  five  wheel  barrow 
loads  of  scale  were  removed  from  this  one  boiler — this  scale 
being  removed  after  the  owner's  operating  engineer  had  re- 
ported the  boiler  clean. 

After  all  the  causes  of  the  troubles  at  this  plant  were 
determined,  it  was  arranged  to  correct  them.  The  results 
obtained  were  astonishing  after  everything  was  fixed  up  and 
a  better  practice  established  in  cleaning  boilers.  The  important 
part  of  the  analysis  is  this :  The  matter  had  to  be  sifted  down 
and  the  causes  of  the  troubles  determined  without  question 
and  presented  to  the  management  of  the  company.  This  cost 
money — who  was  to  pay  for  it  ?  If  someone  had  not  found  the 
troubles  and  expended  the  money  necessary  to  find  them,  the 
result  would  have  been  dissatisfaction  with  the  entire  equip- 
ment; the  stoker  would  have  been  thrown  out;  boilers  con- 
demned; probably  money  unnecessarily  expended  on  a  new 
stack  and  the  owner  convinced  that  some  one  made  a  mistake 
in  the  original  purchase  of  the  equipment. 

In  this  kind  of  work  there  is  always  a  question  of  whether 
or  not  the  trouble  is  due  to  the  stoker  and  boiler  equipment; 
the  operating  conditions,  or  the  design  of  the  breeching  and 
stack.  It  is  necessary,  therefore,  to  make  a  careful  analysis 
and  obtain  sufficient  engineering  facts  that  will  properly  place 
the  reponsibility. 

There  are  many  things  that  effect  a  boiler  and  stoker  com- 
bination and  unfortunately  the  purchaser  or  user  cannot  appre- 
ciate how  anything  wrong  with  a  part  of  the  equipment  can 
effect  another  part. 

A  few  years  ago  the  stoker  manufacturer  had  very  little 
control  over  the  things  that  determined  whether  a  stoker 
installation  would  be  a  success.  As  an  example  of  this,  a 
combination  was  installed,  where  the  stoker  manufacturer 
inquired  as  to  the  height  of  stack  selected  for  the  500  H.P. 
boiler  to  obtain  the  %"  draft  specified  for  the  furnace.  He 
was  told  by  the  architect  that  the  stack  would  be  100  ft.  above 
the  grates.  The  stoker  manufacturer  claimed  that  100  ft.  was 


232  MECHANICAL  STOKERS 

not  enough  and  insisted  on  150  ft.  The  architect  was  greatly 
astonished  that  such  a  height  for  this  one  boiler  was  necessary 
— he  finally  agreed,  however,  to  make  it  125  ft.  The  stoker 
manufacturer  still  insisted  that  a  125  ft.  stack  was  not  sufficient 
and  nothing  less  than  150  ft.  would  do.  Everything  possible 
was  done  to  convince  the  architect  that  this  size  stack  was 
needed.  It  was  finally  decided,  however,  to  build  the  125  ft. 
stack,  the  purchaser  approving  the  architect's  opinion.  The 
stack  was  built  and  sufficient  draft  was  not  obtained  for  the 
amount  of  coal  it  was  necessary  to  burn — consequently  the 
installation  was  a  failure. 

One  stoker  manufacturer  cannot  alone  take  a  firm  stand 
and  hold  out  for  a  better  practice  in  installations,  neither  can 
one  boiler  manufacturer,  but  there  must  be  cooperation  among 
all,  in  order  to  obtain  the  type  of  installation  that  is  required. 

Better  practice  could  be  obtained  if  the  purchaser  gave 
more  thought  to  the  whole  combination  of  boilers,  stokers, 
breechings  and  stacks. 

The  purchase  of  a  stoker  requires  more  thought  than  it  is 
generally  given.  If  a  boiler  and  stoker  combination  is  to  be 
purchased  to  burn  bituminous  coal,  the  type  of  boiler  and 
stoker  should  be  determined  first.  It  will  not  be  attempted 
here  to  outline  the  things  that  must  be  considered  in  deciding 
this  matter.  There  are  all  kinds  of  combinations  that  can  be 
inspected.  Time  can  be  used  to  a  good  advantage  by  finding 
the  good  and  bad  points  of  local  installations  and  thereby  be 
assured  that  contemplated  combinations  will  not  duplicate 
faulty  ones. 

After  the  boiler  and  stoker  has  been  selected,  it  is  not  yet 
time  to  finally  decide  whether  or  not  these  particular  types 
should  be  purchased — it  is  not  the  time  to  sign  the  contracts. 
The  purchaser  should  arrange  a  conference  between  the  manu- 
facturer of  the  boiler  selected,  and  the  manufacturer  of  the 
stoker  selected.  The  whole  combination  and  the  things  men- 
tioned in  the  forepart  of  this  chapter  should  be  discussed  in 
detail  and  a  decision  made  on  each  matter,  final  decision  to 
be  based  on  good  engineering  judgement  and  experience.  The 
purchaser  should  not  obtain  an  opinion  from  the  boiler  manu- 
facturer as  to  how  the  boiler  should  set,  unless  he  gives  the 


INSTALLATION  OF  STOKERS  233 

stoker  manufacturer  an  opportunity  to  present  his  opinions. 
If  all  points  are  settled  on  a  good  engineering  basis,  there  will 
be  no  question  but  what  the  best  combination  known  will  be 
obtained  and  the  customer  can  well  afford  to  expend  the 
money  necessary  for  it.  This  practice  of  buying  stokers  and 
boilers  has  proved  its  effectiveness. 

Many  boilers  and  stokers  are  purchased  by  this  method. 
One  case  is  recalled  where  the  purchaser  decided  on  the  boiler 
and  stoker  that  he  had  in  mind  purchasing;  he  then  called  the 
boiler  manufacturer,  the  stoker  manufacturer  and  his  consult- 
ing engineer  who  was  designing  the  breeching,  stacks,  etc.,  in 
conference  with  him.  He  made  the  statement  that  he  wanted 
the  best  combination  of  this  type  of  boiler  and  stoker  that  he 
could  get  and  wanted  all  conditions  right  for  its  proper  opera- 
tion. At  this  meeting  the  size  of  the  stack  was  decided  on, 
the  setting  of  the  stoker  in  combination  with  the  boiler  was 
arranged,  the  grade  of  firebrick  that  was  to  be  used  in  the 
arches  was  selected;  in  fact,  all  matters,  even  to  the  minutest 
detail  pertaining  to  this  combination,  was  discussed  and  decided 
upon.  After  it  was  thoroughly  understood  between  those 
present  that  in  their  opinion  no  changes  could  be  made  from 
those  decided  that  would  better  the  installation,  the  purchaser 
signed  the  contracts. 

It  is  a  fact  that  this  installation  is  very  satisfactory  in 
every  way.  The  purchaser  of  the  equipment  knew  of  other 
combinations  of  this  same  type  of  boiler  and  stoker  that  were 
failures.  He  also  knew  and  was  capable  of  determining  the 
weak  points  of  those  installations  and  he  made  sure  that  the 
conditions  were  going  to  be  right  for  the  equipment  he  was 
purchasing. 

The  purchaser  must  expend  the  money  necessary  to  obtain 
the  best  combination  known;  he  must  operate  the  equipment 
properly  and  keep  it  in  good  condition.  The  stoker  manu- 
facturer must  take  a  firm  stand  for  the  proper  setting  of  the 
stoker ;  he  must  hold  out  for  the  draft  that  is  required  to  burn 
the  coal  and  he  must  insist  on  the  proper  operation  of  the 
stokers.  The  boiler  manufacturer  must  provide  ample  damper 
areas  and  gas  passages  so  there  will  be  no  restriction  to  the 
flow  of  gases  through  the  boiler  to  the  breeching.  If  these  areas 


234  MECHANICAL  STOKERS 

must  be  changed  to  suit  local  conditions,  then  the  boiler  manu- 
facturer should  advise  the  purchaser  how  they  should  be 
changed.  The  boiler  manufacturer  must  also  provide  easy 
means  for  removing  the  soot  from  the  boiler  tubes  and  baffles ; 
and  he  must  arrange  to  set  the  boiler  according  to  the  require- 
ments of  the  particular  stoker  in  combination. 

This  kind  of  cooperation  is  necessary  for  every  installation. 
Those  installations  that  obtain  it  will  have  very  little  chance 
of  failure. 

STOKER  ENGINEERING  DATA 

Prom  the  forgoing  it  will  be  clear  that  a  stoker  manufacturer 
must  have  certain  pertinent  engineering  data,  from  the  pur- 
chaser in  order  to  study  all  conditions  surrounding  the  installa- 
tion. In  general  the  following  data  cover  a  typical  boiler  room 
case  and  this  information  should  be  given  in  any  specifications 
that  are  proposed  covering  stoker  apparatus: 

Name  and  address  of  Purchaser. 

Location  of  Plant. 

Stoker:   (What  type  of  stoker  are  you  considering?) 

1.  Type. 
Boiler: 

2.  Type Eated  H.P Tubes  high,  or  class,  or  Dia. 

(H.K.T )   Tube  width tube  length 

3.  Center  wall  dimension 

Alleyway  dimension 

Sidewall  dimension 

Furnace  dimension 

Floor  line  to:  Mud  drum  (center  line)  or  Header,  or  shell  (center 

line) 
Stack: 

4.  No.  of  stacks brick  or  steel 

5.  Ht.  above  present  boiler  room  floor. 
Inside  Dia.  at  top. 

Boilers  served Total  H.P.  served 

(each  stack)  (each  stack) 

6.  Connected  to  boiler  by:    Breeching  or  direct. 

Stoker  setting  and  application. 

7.  Old  or  new  boiler?     If  old,   can  they  be  reset?     Can  floor  be 

lowered  ? 


INSTALLATION  OF  STOKERS  235 

8.  What  local  conditions  prevent  the  best  application? 

9.  Will  ashes  be  taken  out:  Floor  or  Basement?     Conveyor  or  Car? 

10.  Do  you  contemplate  installing  future  stokers?    How  many?    Eela- 

tion  to  this  installation? 

11.  What  drawings  do  you  want  for  preliminary  study? 
Fuel  to  be  used: 

12.  Name Where  mined 

13.  F.C Vol Moist Ash Sulphur 

B.T.U.   (as  fired). 
Operating  service: 

14.  Best  economy  desired  at rating.  What  will  be  maximum 

rating?     Duration  (hrs.)     Avg.  daily  rating. 

15.  Eemarks  regarding  the  character  of  service  and  plant  organization. 

16.  Steam  pressure  of  plant Superheat Back  pressure 

Stoker  Drive:    (Do  you  want  stoker  driven  by) 

17.  Engine 

Turbine  and  Gear 

Motor 

Forced  Draft   Fan  Equipment:    (How  many  fans  do  you  want,  and  how 
driven?) 

18.  Number  of  Fans  (   ) 
Driven  by 

(   )  Turbine  and  Gear. 
(   )   Motor. 
(   )  Engine. 
Deliveries  Required: 

19.  Stokers  and  Equipment Months. 

Drawings : 

20.  If  possible,  furnish  the  following  drawings  with  specification: 

(1)  Boilers  and  boiler  settings  as  installed. 

(2)  Beams  and  columns  adjacent  to  boilers. 

(3)  Details  of  auxiliary  equipment,  piping,  etc.,  that  may  inter- 

fere with  stoker  settings. 

In  drawing  specifications  for  stokers  it  is  quite  essential 
that  they  be  specific  and  contain  the  engineering  data  required. 
Following  is  a  good  type  of  specification  in  general  use. 


236  MECHANICAL  STOKERS 

SPECIFICATIONS 

for 
MECHANICAL  STOKING  EQUIPMENT 

for  the 

PITTSBURGH  ELECTRIC  COMPANY 
PITTSBURGH,   PA. 

Pittsburgh,  Aug.  16,  1920. 

General  Data. — These  specifications  are  intended  to  cover 
the  requirements  for  Mechanical  Stoking  and  Forced  Draft 
Equipment  to  be  constructed,  delivered,  and  erected  complete 
with  all  appurtenances  and  otherwise  ready  for  service,  started 
and  put  in  successful  operating  condition  by  Contractor  in 
the  DuQuesne  Station,  located  at  First  Avenue  and  Forty- 
Sixth  Street,  Pittsburgh,  Pa.,  of  the  Pittsburgh  Electric  Com- 
pany, Pittsburgh,  Pa.,  hereinafter  called  the  Purchaser. 

Contractor  may  deliver  his  material  to  the  station  over  a 
siding  from  the  P.  R.  R.,  which  enters  the  boiler  room. 

Boiler. — The  twelve  boilers  under  which  Contractor  shall 
erect  the  stokers  are  A.  &  C.  Company's  all  wrought  steel  con- 
struction, arranged  two  in  battery,  each  boiler  containing 
approximately  6000  sq.  ft.  of  water  heating  surface.  The 
boilers  are  21  tubes  wide  and  14  tubes  high  with  3-42"  drums, 
are  12'  1"  wide  inside  of  setting  walls,  are  set  10  ft.  high  from 
floor  line  to  bottom  of  front  header  and  are  equipped  with  125° 
F.  Atlas  Superheaters. 

Working  Pressure. — Under  normal  operating  conditions  the 
boilers  will  deliver  steam  at  200  Ibs.  per  sq.  inch  gauge  pressure 
and  superheated  at  125°  F. 

Stacks. — Boilers  number  2,  4,  6,  8,  10  and  12  are  served  by 
a  brick  stack  13'  6"  clear  diameter  by  193'  above  boiler  room 
floor  and  located  adjacent  to  the  East  side  of  the  boiler  room. 

Boilers  number  1,  3,  5,  7,  9  and  11  will  be  served  by  a  brick 
lined  steel  stack  15'  6"  clear  diameter  by  200'  above  boiler 
room  floor  and  located  immediately  over  future  boilers  number 
13  and  15. 

Soot  Blowers. — Each  boiler  will  be  equipped  with  soot 
blowers. 


INSTALLATION  OF  STOKERS  237 

Coal. — The  stoking  equipment  furnished  hereunder  shall  be 
constructed  and  guaranteed  by  Contractor  to  burn  success- 
fully and  economically  at  all  rates  within  and  including  the 
maximum  safe  operating  capacity  of  its  feeding  mechanism, 
steam  coals  common  to  the  Pittsburgh  market,  among  which 
the  principal  is  Youghiougheny  screenings  of  approximately 
the  following  analysis : 

Volatile   combustible   matter 30.81% 

Fixed  carbon  56.46% 

Sulphur   1.70% 

Ash  11.03% 

B.T.U 13,400% 

Operation. — After  the  first  two  stoking  equipments  are 
erected  and  ready  for  operation,  Contractor  shall  furnish  and 
shall  maintain  daily  at  the  Station  for  a  period,  of  six  weeks, 
a  competent  firemen  thoroughly  experienced  and  skilled  in  the 
operation  of  the  stoker  equipment  furnished  by  him.  Said 
fireman  shall  direct  the  operation  of  said  stoking  equipment 
and  shall  carefully  and  thoroughly  instruct  Purchaser's 
employes  in  the  proper  care,  manipulation  and  operation  there- 
of, to  the  end  that  Purchaser  may  derive  the  greatest  benefit 
from  the  installation. 

Stoking  Equipment. — Apparatus  to  be  furnished  and  erected 
hereunder  by  Contractor  consist  of  the  following: 

Twelve  mechanical  stokers  of  identical  construction,  except 
that  six  will  be  left  hand  and  six  will  be  right  hand. 

The  necessary  new  lower  half  boiler  fronts,  stoker  bearing 
bars,  anchor  bolts,  buck  stays,  driving  gear  regulators,  clean- 
out  and  inspection  doors  and  all  other  appliances,  attachments 
and  apparatus  necessary  to  make  the  installation  complete  and 
ready  for  commercial  service. 

Purchaser  will  remove  lower  half  fronts  and  grates  on 
boilers  now  in  place  and  will  furnish  and  set  in  place  all 
necessary  structural  steel  work  and  other  foundations  for 
carrying  the  stoker  framework  under  the  direction  of  the 
Contractor's  erecting  superintendent. 

Purchaser  will  do  all  necessary  cutting  away  and  rebuild- 
ing of  boiler  settings  and  furnace  brickwork. 


238  MECHANICAL  STOKERS 

Purchaser  will  provide  suitable  foundations  and  foundation 
bolts  for  forced  draft  equipments. 

Purchaser  will  furnish  all  steam  and  exhaust  piping  out- 
side of  the  stop  valves  and  exhaust  openings  on  Contractor's 
apparatus. 

Purchaser  will  furnish  and  erect  the  air  ducts  and  con- 
nections to  the  stoker  wind  boxes  of  ample  area  which  will  be 
arranged  as  indicated  on  drawing,  attached  to  and  forming 
part  of  these  specifications. 

Capacity  and  Efficiency. — Contractor  shall  state  in  his  pro- 
posal the  following: 

1.  The  safe  continuous  maximum  coal  feeding  capacity  per 
hour  of  each  stoker  offered  by  him. 

2.  When   burning  Youghiougheny   screenings   of   approxi- 
mately the  heating  value  specified  above,  the  maximum  capacity 
and   corresponding  combined  efficiency  each   equipment  will 
develop  continuously  from  the  boiler  under  which  it  is  installed. 

(a)  For  a  period  of  36  consecutive  hours. 

This  capacity  must  not  be  less  than  1200  boiler  horse  power. 

(b)  For  a  period  of  8  consecutive  hours. 

(c)  For  a  period  of  2  consecutive  hours. 

This  capacity  must  not  be  less  than  1800  boiler  horse  power. 

3.  The  combined  efficiency  when  developing  continuously 
the  following  capacities: 

(a)  600  boiler  horse  power. 

(b)  900      " 

4.  The  pounds  of  coal  per  hour  or  B.T.U.  per  hour  required 
to  be  fed  to  stoker  to  maintain  on  a  banked  boiler  a  steam 
pressure  of  190  Ibs.  per  sq.  inch  with  steam  stop  valve  closed. 

5.  Time  required  to  develop  from  a  boiler  so  banked  for  a 
period  of  48  hours  a  capacity  of  (a)  600  boiler  horse  power  (b) 
1200  boiler  horse  power. 

6.  Steam  consumption  in  pounds  per  hour  of  the  stokers  and 
of  the  forced  draft  apparatus  when  the  boilers  served  by  it  are 
operating  at : 

(a)  600  boiler  horse  power. 

(b)  1200      " 

(c)  Maximum  (2-hour  rate). 


INSTALLATION  OF  STOKERS  239 

Mechanical  Stokers.— Stokers  shall  be  of  the  inclined  under- 
feed  type  and  shall  be  provided  with  adequate  blast  boxes,  air 
openings  and  tuyeres  for  conducting  the  air  required  for  com- 
bustion properly  to  all  parts  of  the  bed  of  fuel  and  discharging 
it  into  the  bed  of  fuel  from  beneath.  The  blast  boxes  and  air 
openings  shall  be  so  proportioned  and  separated  into  sections, 
each  provided  with  suitable  damper,  that  the  air  pressure  in  the 
fuel  bed  can  be  adjusted  as  may  be  required  for  best  combus- 
tion and  most  satisfactory  manipulation  of  the  fire.  These 
dampers  shall  be  arranged  to  be  conveniently  operated  from 
the  front  or  side  of  the  stokers  and  so  provided  with  suitable 
locking  device  that  they  will  remain  in  position  when  set. 

Hoppers. — Each  stoker  shall  be  provided  with  a  suitable 
coal  hopper  extending  its  entire  width  and  made  of  sheet  steel 
rigidly  braced. 

These  hoppers  shall  be  designed  to  receive  coal  from  an 
overhead  chute  and  deliver  it  to  the  feeding  plungers.  Hoppers 
shall  have  ample  capacity  to  contain  the  quantity  of  coal 
required  for  a  period  of  not  less  than  10  minutes  when  the 
stokers  are  feeding  coal  at  the  maximum  rate. 

Retort  and  Feeding  Mechanism. — The  plungers  shall  be 
arranged  for  receiving  coal  from  the  hoppers  and  delivering  it 
to  the  retorts.  Retorts  shall  be  of  cast-iron,  of  substantial  con- 
struction and  so  designed  as  to  be  free  from  warping  or  defor- 
mation under  the  normal  conditions  of  service  and  free  from 
roughness  or  ridges  which  might  obstruct  the  passage  of  the 
coal.  Suitable  ram  blocks  shall  be  provided  in  the  retorts  to 
insure  even  distribution  of  the  coal.  Travel  of  ram  blocks  and 
moving  grates  shall  be  adjustable  while  stokers  are  in  opera- 
tion. 

Operating  Shafts  and  Auxiliary  Mechanism. — The  plungers, 
ram  blocks  and  moving  grates  shall  be  given  a  reciprocat- 
ing motion  by  suitable  rods  connecting  with  cranks  on  the 
drive  shaft.  The  rod  ends  which  connect  with  the  cranks  shall 
be  of  the  marine  type  and  fitted  with  genuine  babbitted  bush- 
ings. The  drive  shaft  shall  be  of  cast  steel  with  journals 
turned  true  and  accurately  in  line.  Shaft  bearings  shall  be 
rigidly  attached  to  the  stoker  front  by  suitable  brackets  of 
ample  strength  and  free  from  deflection.  Shaft  bearing  boxes 


240  MECHANICAL  STOKERS 

shall  be  genuine  babbitt  lined.  Shaft  bearings  and  connecting 
rod  bearings  shall  be  arranged  for  grease  lubrication  and  grease 
cups  shall  be  furnished.  Power  shall  be  transmitted  to  the  drive 
shaft  by  a  suitable  cut  worm  and  cut  gear  mechanism  running 
in  oil  in  a  dust  and  oil  tight  cast-iron  box  having  removable 
cover  for  inspection  and  adequate  means  for  removing  and 
renewing  oil. 

Dumping  Grates. — Each  stoker  shall  be  equipped  with  an 
adequate  dumping  grate.  Dumping  grate  shall  be  easily 
operated  by  levers  suitably  located  in  front  of  the  boilers. 
Dumping  grate  shall  be  arranged  to  prevent  the  accumulation 
of  clinkers  on  the  bridge  wall  or  on  other  parts  that  might 
choke  or  hinder  the  proper  dumping  of  ashes,  etc. 

The  ashes  and  clinkers  will  fall  into  an  ash  hopper  located 
below  the  furnace  from  which  they  will  be  delivered  by  gravity 
into  the  ash  conveying  equipment  for  removal.  The  discharge 
openings  on  the  ash  hoppers  will  be  about  24"  square.  Adequate 
provision  must  be  made  by  Contractor  in  the  construction  and 
arrangement  of  the  dumping  grates  so  that  the  clinkers  dis- 
charged by  his  stoker  when  properly  operated  shall  be  small 
enough  to  discharge  freely  and  easily  from  the  ash  hoppers 
in  the  manner  described  without  the  necessity  for  continual 
punching,  raking,  stirring  or  breaking  up  clinkers  in  the  ash 
pit. 

Smoke. — The  stoking  equipment  furnished  hereunder  shall 
operate  under  all  conditions  of  load  and  fuel  so  as  to  conform 
to  the  smoke  ordinances  of  the  City  of  Pittsburgh. 

Forced  Draft  Equipment. — Contractor  shall  furnish  four 
sets  of  forced  draft  equipment,  any  three  of  which  shall  be 
easily  capable  of  delivering  continuously  a  sufficient  quantity 
of  air  at  a  sufficient  pressure,  including  ample  allowance  for 
friction  losses  in  air  ducts,  damper  boxes,  etc.,  to  develop  from 
the  twelve  boilers  simultaneously  the  maximum  capacity  at 
the  2-hour  rate  guaranteed  by  him  for  each  boiler. 

The  forced  draft  equipments  shall  be  of  similar  construction 
and  shall  be  driven  by  steam  turbines  of  manufacture  approved 
by  the  purchaser.  Turbines  shall  operate  non-condensing  with 
normal  steam  pressure  of  200  Ibs.  per  sq.  inch,  superheated 
about  140°  F.,  but  they  shall  be  designed  and  arranged  with 
suitable  by-pass  valves  hand  operated,  as  to  be  easily  capable 


INSTALLATION  OF  STOKERS  241 

of  developing  the  maximum  capacity  specified  above  when 
supplied  with  steam  at  150  Ibs.  per  sq.  inch  by  gauge  and 
superheated  100°  F. 

The  exhaust  pressure  will  not  exceed  one  pound  per  square 
inch  above  atmospheric  pressure. 

Each  turbine  shall  be  equipped  with  safety  governor  ade- 
quate to  prevent  development  of  a  dangerous  speed,  also  a 
suitable  governor  to  maintain  a  practically  uniform  speed  under 
all  conditions  and  changes  of  load.  Speed  governors  shall  be 
capable  of  adjustment  so  that  there  will  be  no  interference  or 
"bucking"  when  two  or  more  fan  units  are  operating  together 
feeding  into  the  main  air  ducts. 

Contractor  shall  state  and  guarantee  the  maximum  con- 
tinuous capacity  and  blast  pressure  of  each  of  the  forced  draft 
equipments  offered  by  him  and  the  speed  of  the  equipment 
when  operating  under  such  maximum  conditions. 

Air  Ducts. — Purchaser  will  furnish  and  erect  the  necessary 
air  duct  (a)  between  the  fans  and  the  main  air  duct  and  (b) 
between  the  main  air  duct  and  the  blast  boxes  on  the  stokers. 
All  air  ducts  will  be  constructed  of  No.  16  Birmingham  wire 
gauge  galvanized  iron;  joints  will  be  substantially  riveted 
together  and  soldered  air  tight  after  riveted. 

Air  ducts  between  fans  and  main  air  duct  will  be  fitted 
with  hand  operated  butterfly  dampers  with  suitable  lever 
handle  and  provided  with  suitable  locking  device  that  they 
will  remain  in  position  when  set. 

Air  ducts  between  main  air  duct  and  stoker  blast  boxes 
will  be  fitted  with  butterfly  dampers  operated  by  suitable 
regulating  devices  specified  below. 

Driving  Mechanism. — Contractor  shall  furnish  suitable  driv- 
ing mechanism  for  stokers.  Stokers  shall  be  driven  either  by 
belt  or  chain  from  2  line  shafts  furnished  and  erected  by  Con- 
tractor, one  for  each  side  of  the  boiler  room,  complete  with 
all  necessary  hangers,  brackets,  bearings,  etc. 

Each  stoker  line  shaft  shall  be  arranged  with  jaw  clutch 
between  each  battery  of  boilers  and  shall  be  driven  through 
jaw  clutches  by  two  engines  of  manufacture  and  construction 
approved  by  the  Purchaser. 

Each  engine  shall  be  of  ample  capacity  to  drive  all  stokers 


242  MECHANICAL  STOKERS 

on  its  line  shaft  at  maximum  capacity  when  supplied  with 
steam  at  150  Ibs.  per  sq.  inch  gauge,  and  exhausting  against  1 
Ib.  back-pressure  by  gauge,  but  shall  be  designed  for  normal 
operation  at  the  normal  operating  steam  pressure  of  the  station. 

Each  engine  shall  be  fitted  with  adequate  governor  arranged 
to  operate  to  prevent  over-speed  only.  Speed  of  the  engine  in 
normal  operation  shall  be  controlled  by  regulating  devices 
specified  below. 

Regulating  Devices. — Contractor  shall  furnish  suitable  auto- 
matic devices  for  regulating  the  supply  of  coal  and  air  to  the 
stokers  in  accordance  with  the  demand  for  steam.  The  regulat- 
ing devices  shall  be  adjusted  to  maintain  at  all  times  such 
a  proportion  between  the  coal  and  air  as  will  develop  the 
highest  practicable  combustion  efficiency  and  maintain  the 
steam  pressure  within  2%  Ibs.  of  the  normal  operating  pressure 
at  all  capacities  including  the  maximum. 

It  is  proposed  to  operate  the  fans  at  constant  pressure  and 
control  the  air  supply  to  the  fuel  bed  by  automatically  operat- 
ing the  butterfly  dampers  in  the  air  ducts  between  the  main 
air  duct  and  the  stoker  blast  boxes.  Contractor  shall  furnish 
for  this  purpose  a  damper  regulator  approved  by  the  Purchaser, 
said  regulator  being  operated  directly  by  the  change  of  steam 
pressure  due  to  the  demand  for  steam. 

It  is  proposed  to  control  the  supply  of  fuel  to  the  furnace 
by  regulating  the  speed  of  the  engine  operating  the  stoker  line 
shaft.  For  this  purpose  Contractor  shall  furnish  suitable 
regulating  device  of  type  approved  by  the  Purchaser,  said 
regulator  being  operated  by  the  steam  pressure  on  the  station, 
by  the  air  pressure  in  the  stoker  blast  boxes,  or  by  such  other 
means  approved  by  the  Purchaser  as  Contractor  may  recom- 
mend. 

The  regulating  devices  shall  be  of  simple  and  substantial 
construction  easily  adjusted,  arranged  to  maintain  adjustments 
when  once  set,  and  connected  so  that  they  may  be  easily  and 
rapidly  thrown  into  and  out  of  operation  should  it  at  any  time 
be  desirable  to  regulate  by  hand. 

Contractor  shall  include  in  his  proposal  complete  descrip- 
tion of  the  regulating  devices  he  proposes  to  use  for  these 


INSTALLATION  OF  STOKERS  243 

purposes  and  show  therein  how  change  in  the  resistance  of  the 
fuel  bed  due  to  varying  size  of  coal  is  provided  for. 

Description  and  Drawing's. — Contractor  shall  include  with 
his  proposal  a  complete  detailed  description  and  completely 
dimensioned  general  arrangement  drawings  of  the  apparatus 
offered  by  him.  After  the  contract  is  let  Contractor  shall 
furnish  promptly  complete  detailed  drawings  of  the  apparatus 
to  be  furnished  by  him  hereunder. 

Tests. — At  a  convenient  time  not  more  than  six  months 
after  completion  by  Contractor  of  his  work  hereunder,  Pur- 
chaser will  make  such  service  and  running  tests  as  considered 
necessary  to  determine  whether  the  equipment  furnished  here- 
ander  conforms  to  the  requirements  of  these  specifications. 
Contractor  shall  be  notified  in  advance  of  such  tests  and  may 
have  a  representative  present.  Should  the  tests  show  that  the 
equipment  furnished  by  him  does  not  meet  the  requirements 
of  these  specifications,  Contractor  shall  promptly  and  at  his 
own  expense  make  the  necessary  changes  to  make  it  conform 
thereto. 

STOKER  GUARANTEES 

A  stoker  primarily  is  for  burning  fuel  and  delivering  the 
resultant  heat  to  a  boiler  or  other  kind  of  vessels.  How  well 
a  stoker  does  its  work  is  theoretically  based  on  the  following: 

(1)  Pounds  of  coal  burned  per  hour. 

(2)  Percent  of  combustible  in  ash. 

(3)  Percent  of  CO2  in  furnace  gases. 

(4)  Percent  of  CO  in  furnace  gases. 

(5)  Draft  required  in  the  furnace. 

The  purchaser  however,  wants  to  know  how  many  pounds 
of  steam  can  be  obtained  from  a  pound  of  coal.  Consequently, 
stoker  manufacturers  are  asked  to  make  guarantees  that  are 
affected  by  the  following: 

(1)  Type  of  boiler. 

(2)  Condition  of  brick  setting. 

(3)  Layout  and  design  of  breeching  and  stack. 

Stoker  manufacturers  therefore  do  not  approve  of  making 
an  overall  guarantee,  but  in  order  to  specify  performances  it 
is  important  that  they  be  given  all  the  engineering  data  pre- 


244  MECHANICAL  STOKERS 

viously  asked  for.    For  general  purposes,  the  following  per- 
formances are  generally  given  to  Purchasers : 

(1)  Maximum  percent  of  rating  that  can  be  obtained  from  the  boilers 

for  24  hours  duration. 

(2)  Maximum  percent  of  rating  that  can  be  obtained  from  the  boilers 

for  short  duration — to  carry  over  peaks. 

(3)  Combined  boiler  and  grate  efficiency  at  the  boiler  rating  that  boilers 

will  be  operated  at  the  greatest  number  of  hours  during  the  day. 

TYPICAL  STOKER  CONTRACT  FORMS 

Most  stoker  manufacturers  sell  their  product  on  a  proposal 
form,  which  is  submitted  to  the  purchaser  and  if  accepted  by 
him  and  approved  by  the  manufacturer  becomes  a  contract. 
The  typical  form  used  is  as  follows : 

THE  IDEAL  STOKER  COMPANY. 

CONTRACT  PROPOSAL  No Detroit,  Mich., 192. . 

To 

Hereinafter  called  the  Purchaser. 

P.  O.  Address 

Shipping  Address   

For  Stoker  equipment  for  use  in  connection  with  the  following: 

(A  rated  boiler  horse  power  being  taken  as  10  sq.  ft.  of  effective  water- 
heating  surface)   


(1)  THE  IDEAL  STOKER  COMPANY  (hereinafter  called  the  contractor) 
proposes   to   furnish  the   Purchaser,   f.o.b.   cars  point  of  shipment, 
apparatus  as  specified  below: 
(a)  Stokers: 
(&)  Actuation: 

(c)   Stoker  companies  arrange  A,  B,  C,  etc.,  for  their  individual  equip- 
<£     ment  requirements. 


(2)  FURNISHED  BY  THE  PURCHASER: 

The  Purchaser  agrees  to  unload  all  mechanical  stoker  equipment  and 
accessories  herein  specified  to  be  furnished  by  the  contractor,  and  to  place 
same  adjacent  to  the  foundations  upon  which  the  said  equipment  and 
accessories  are  to  be  erected.  All  materials,  boxed  or  otherwise,  are  to 
be  protected  from  the  weather. 

The  Purchaser  will  furnish  labor  and  tackle  for  erecting  all  equipment. 


INSTALLATION  OF  STOKERS  245 

The  Purchaser  will  furnish,  in  place,  all  foundations,  anchorage  and 
anchor  bolts;  steel  and  concrete  pits,  air  duct,  ash-pit  doors,  supports  for 
line  shaft  hangers,  boilers  and  boiler-setting  walls,  draft  gauges,  all  steam 
piping,  electric  wiring,  and  all  other  equipment  not  otherwise  specified 
herein. 

(3)  SUPEEINTENDENCE: 

Unless  otherwise  stipulated,  all  materials  shall  be  installed  by  and  at 
the  expense  of  the  purchaser  and  under  the  supervision  and  direction  of 
a  Superintendent,  to  be  furnished  by  the  Contractor,  for  whose  services 

the  Purchaser  agrees  to  pay  the  Contractor  the  sum  of 

($ )    dollars  per  calendar  day    (which  shall  include  time  traveling) 

plus  living  and  traveling  expenses,  all  of  which  shall  be  paid  by  the 
Purchaser  as  invoices  are  presented;  it  being  understood  and  agreed  that 
during  the  term  of  such  services,  the  Superintendent  shall  be  the  Pur- 
chaser's employee. 

(4)  PEEFOEMANCE  CONDITIONS: 

It  is  understood  and  agreed  that  any  guarantees  are  based  upon  the 
Purchaser  providing  the  following  conditions: 

(a)  That  the  stoker  equipment  has  been  erected  in  accordance  with  the 
Company's  plans  and  specifications,  and  is  properly  operated. 

(b)  That  the  boilers  shall  be  of type  with  a  minimum   distance 

from  heating  surface  of  boiler  to  grate  of ft. 

(c)  That  the  boilers  are  in  good  condition  and  heating  surface  clean, 
inside  and  out. 

(d)  That  the  baffling  is  tight  and  so   arranged  that  the  temperature 
of  the  flue  gases  at  boiler  outlet  shall  not  exceed  550  degrees  F.  when  the 
boiler  is  operated  at  150%  of  nominal  rating. 

(e)  That   the   boiler   setting   and   furnace   brickwork   are    in   first-class 
condition  and  free  from  excessive  air  leaks,  as  determined  by  candle  flame, 
in  accordance  with  A.  S.  M.  E.  Boiler  Test  Code. 

(/)  That  the  draft  (negative  pressure)  provided  by  the  Purchaser  and 

available  in  the  furnace  of  each  stoker,  shall  not  be  less  than inches 

water  column  when  boiler  is  operating  at  maximum  rating  specified. 

(#)  There  shall  be  available  for  each  stoker,  when  boiler  is  operating 

at  maximum  rating  specified,  not  less  than cubic  feet  of  air  per 

minute  referred  to barometer,  and  70  degrees  Fahr.  at inches 

water  column  static  pressure  at  stoker  forced-draft  inlet. 

(ft)  That fuel,  known  commercially  as size and 

of  the  following  proximate  analysis  shall  be  used: 

%  Fixed  Carbon 

%  Volatile   Matter 

%  Ash 

%  Moisture 

%  Sulphur  (Sep.  Det.) 


246  MECHANICAL  STOKERS 

B.T.U.  per  pound  as  fired  not  less  than 

Fusing  temperature  of  ash  not  less  than Fahr. 

(5)  DATE  OF  SHIPMENT: 

The  Contractor  agrees  (unless  delayed  by  the  Purchaser)  that  shipments 
will  be  made days  after  acceptance  of  this  proposal. 

The  Purchaser  shall  furnish  the  Contractor,  within  ten  days  from  date 
of  acceptance  of  this  proposal,  all  data  necessary  to  enable  the  Contractor 
to  Complete  his  drawings.  The  Contractor  shall  be  privileged  to  extend 
date  of  shipment  specified  without  notice,  in  case  the  Purchaser  delays 
furnishing  information  necessary  to  complete  the  Contractor's  drawings, 
or  delays  in  approving  same.  The  Contractor  shall  be  granted  reasonable 
time  after  final  drawings  are  approved  to  complete  material  lists  and 
arrange  for  shipment.  The  acceptance,  when  delivered,  of  the  mechanical 
stoker  equipment  herein  specified  to  be  supplied  by  Contractor,  shall  con- 
stitute a  waiver  of  all  claims  for  damages  caused  by  any  delay. 

(6)  CONSEQUENTIAL  DAMAGE: 

The  Contractor  shall  not  be  held  liable  for  any  loss,  damage,  detention 
or  delay  caused  by  fires,  strike,  civil  or  military  authority,  or  by  insurrection 
or  riot,  or  by  any  other  cause  which  is  unavoidable  or  beyond  its  reasonable 
control,  or,  in  any  event,  for  consequential  damages. 

(7)  EEPLACEMENT  OF  DEFECTIVE  PAKTS: 

The  Contractor  agrees  to  correct,  and  shall  have  the  right  to  correct, 
by  supplying  new  parts  at  its  own  expense,  f.o.b.  Point  of  Shipment,  any 
defects  in  the  apparatus  furnished  by  the  Contractor  which  may  develop 
under  normal  and  proper  use,  within  one  year  from  the  shipment  thereof 
provided  the  Purchaser  gives  the  Contractor  immediate  written  notice  of 
such  defects,  and  correction  of  such  defect,  by  supplying  such  parts  by  the 
Contractor,  shall  constitute  a  fulfillment  of  all  his  obligations  to  the 
Purchaser  hereunder. 

Acceptance  by  Purchaser  IDEAL    STOKEE    COMPANY 

The    foregoing   proposal    is    hereby      gy 
accepted  and  agreed  to  this.  .    .  .day      Approved  at 

This day  of 192... 

(Purchaser  to  sign 'here')'  '  IDEAL  STOKER  COMPANY? 

By By. 

SPECIFICATIONS  FORMS 

Every  stoker  builder  has  certain  specifications  for  the  par- 
ticular type  of  stoker  sold,  but  in  general  these  forms  cover 
about  the  same  items  as  follows : 


INSTALLATION  OF  STOKERS  247 


CHAIN  GRATE  STOKERS 

The  following  items  are  'named  in  that  part  of  the  proposal 
covering  materials  required. 

1.  General  description  of  stoker. 

2.  Equipment  furnished  by  Contractor. 

(1)  Number  and  size  of  stokers. 

(Sq.  Ft.  grate  surface  and  weight  within  5%) 

(2)  Number  and  size  of  ignition  arches. 

(3)  Number  and  size  of  upper  arches. 

(4)  Type  of  water  backs. 

(5)  Inspection  doors. 

(6)  Kails. 

(7)  Ledge  Plates. 

(8)  Arch  cover  Plates. 

3.  Stoker  drive  furnished  by: 

(1)   Number,  size,  types  of  prime  movers,  pulleys,  shafts,  hangers 
and  boxes. 

4.  Forced  draft  furnished  by: 

(1)  Description  and  method  of  operation. 

(2)  Eegulation. 

5.  Purchaser  will  furnish. 

(1)  All  masonry,  foundations,  grouting  foundation,  bolts  and  sup- 

porting structure. 

(2)  All  labor  for  unloading  and  erection  of  stoker  equipment. 

(3)  All  ash  pits  and  ash  pit  construction. 

(4)  Complement  of  gauges,  for  draft,  air  pressure,  etc. 

(5)  All  piping,  valves,  wiring  and  other  connections. 

6.  Optional  Equipment. 

(1)  Curtain  wall  supports  furnished  by  purchaser  or  contractor. 

(2)  Lower  boiler  fronts  furnished  by  purchaser  or  contractor. 

(3)  Drip  pans  for  header  furnished  by  purchaser  or  contractor. 

7.  Boilers. 

(1)  Number  and  type. 

(2)  Horse  power. 

(3)  Sq.  Ft.  heating  surface. 

(4)  Width  and  length  furnace. 

(5)  Setting  height,  single  or  battery  setting. 

(6)  Steam  pressure  and  superheat. 


248  MECHANICAL  STOKERS 

BOILEK  BOOM  LOG 

I.  Installation  Data: 

1.  Plan  of  Boiler  room. 

2.  Setting  Details. 

3.  Breeching  Details. 

4.  Stack  Details. 

5.  Coal  handling  Machinery  Capacities. 

6.  Ash  handling  Machinery  Capacities. 

7.  Oil  burning  equipment  details. 

8.  Stoker  details. 

9.  Piping  details. 
10.  Heater  details. 

II.  Labor  Data — Boiler  Koom: 

1.  Engineers  Numbers  Hrs.  Eates  Schedules  Field. 

2.  Firemen 

3.  Water  Tenders         " 

4.  Coal  Passers 

5.  Ash  Handlers  "  "         "  " 

6.  Boiler  Washers 

7.  Janitors 

8.  Specialists  "  "         "  "  " 

9.  Apprentices  "  "          "  "  " 

10.  Chart  showing  responsibility  and  authority.     Sec.  III.  10. 

III.  Labor  Data — Engine  Koom: 

1.  Engineers  Numbers  Hrs.  Eates  Schedules  Field. 

2.  Oilers  "  "         "  " 

3.  Electricians 

4.  Janitors 

5.  Eepairmen 

6.  Apprentices 
7. 

8. 
9. 
10.  Chart  showing  responsibility  and  authority.     Sec.  II,  10. 

IV.  Maintenance  Data — Boiler  Eoom — One  Year: 

1.  Brickwork  Men — Hrs.  Eates  Materials 

2.  Boiler  Tubes                  "  "  "    (individual  Boilers) 

3.  Boiler  Trimmings          "  "  " 

4.  Auxiliaries                       "  "  "           " 

5.  Stokers 

6.  Oil  Equipment                "  "  ll           " 

7.  Feed  Water  Heaters    "  "  "           " 

8.  Ash  Handling                 "  "  "           " 

9.  Coal  Handling 
10.  Building  Eepairs 


INSTALLATION  OF  STOKERS  249 

V.  Coal  Data: 

1.  Quantities  by  days,  months,  year.    Curve. 

2.  Daily  consumption  curve  by  hrs.  Add  VI  2  on  heat  equiv. 

3.  Heat  values  per  Ib. 

4.  Proximate  analyses. 

5.  Bate  of  combustion  per  sq.  ft.  per  hr.  Max.-Min.    Sec.  VII, 

6,  7,  8. 

VI.  Oil  Data: 

1.  Quantities  by  days,  months,  year.    Curve. 

2.  Daily  consumption  curve  by  hours.    Add  V  2  on  heat  equiv. 

3.  Heat  values  per  Ib. 

4.  Proximate  or  Ultimate  analyses. 

5.  Bate  of  combustion  combine  with  V  5  on  heat  equiv.  basis  in 

curves. 

VII.  Boilers  in  Service 

1.  Total  number  in  service  daily— Chart  for  mo.  Averages  per  yr. 

2.  Maximum  capacity  (B.H.P.)  per  boiler  unit. 

3.  Minimum  capacity  (B.H.P.)  per  boiler  unit. 

4.  Number  units  operating  max.  cap.  on  peak  load. 

5.  Number  units  operating  min.  cap.  on  valley  load. 

6.  Total  max.  B.H.P.  developed  at  peak  load— Bate  of  combus- 

tion. Sec.  VII  8. 

7.  Total  min.  B.H.P.  developed  at  valley  load — Bate  of  combus- 

tion. Sec.  VII  8. 

8.  Total  grate  area  in  service  daily — Chart  per  mo.  Avgs.  per  yr. 

9.  Time  in  hours  to  get  up  steam  from  cold  boiler. 

10.  Description  of  banked  fire. 

11.  When  and  how  is  oil  used. 

VIII.  Electrical  Output  Data: 

1.  Characteristic  Load  (K.W.H.)  Curves— Daily. 

2.  Characteristic  Load  (K.W.H.)  Curves— Monthly. 

IX.  Water  Data: 

1.  Purification  System  and  Methods — Baw  Water  Analyses. 

2.  Meter  Monthly  Beadings — Daily — Monthly  Charts. 

3.  Steam  charts — Daily — Monthly. 

X.  Temperature  Data— Boiler  Boom: 

1.  Feed  Water. 

2.  Furnace. 

3.  Uptake. 

4.  Before  and  after  economizer — Gas  side. 

5.  Before  and  after  economizer — Water  side. 

6.  Superheat — actual  and  increase. 

7.  Stack  Base. 

8.  Boiler  Settings — Charts. 

9.  Boiler  Boom. 

10.  Breeching  at  various  points. 


250  MECHANICAL  STOKERS 

XI.  Pressure  Data: 

1.  Steam  at  Boiler — charts. 

2.  Fall  through  superheaters. 

3.  Fall  to  engines. 

4.  Draft. 

a.  Stack  Base. 

fe.  Breeching  at  various  points. 

c.  Boiler  Setting  throughout — chart.     To  be  studied  in  con- 

junction with  X  2,  3,   10. 

d.  Between  interior  and  exterior  boiler  rooms. 

e.  Source  of  air  supply. 

5.  Oil  Equip. 

a.  Steam  pressure  at  burner, 
fc.  Oil  presure  at  burner. 

XII.  Flue  Gas  Data: 

1.  Continuous  C.O.  charts. 

2.  Flue  Gas  analyses  at  uptakes. 

3.  Flue  Gas  throughout  setting  simultaneous  with  XII,  2. 

4.  Determinations  of  air  leakages  based  on  XII,  3. 

XIII.  Eoutine  Service — Boiler  Boom: 

1.  Soot  blowing,  Manner,  Time,  Duration,  By  Whom — Effective- 

ness. 

2.  Soot  deposits. 

a.  Frequency  of  removal, 
fe.  Quantity  of  removal. 
c.  Inspected  by  whom. 

3.  Boiler  Washing. 

a.  Schedule. 

fc.  Time  taken  for  each  boiler  unit. 

c.  Method. 

d.  By  whom  inspected. 

4.  Setting  Overhauling. 

a.  Air  leakage  tests. 
fc.  Baffle  leakage  tests. 

c.  Boiler  covering. 

d.  Inspected  by  whom. 

5.  Blow  Downs. 

a.  Frequency — Time  of  Day — By  Whom. 
5.  Duration. 

XIV.  Smoke  Data: 

1.  Continuous  records. 

2.  Method  used  in  judging  smoke  density. 

3.  Causes  of  smoke  production. 

4.  Meaning  of  periods  when  smoke  is  absent — Sec.  XII,  2. 

5.  Time  when  most  prevalent. 


INSTALLATION  OF  STOKERS  251 

XV.  1.  Heat  value  chart  showing  total  supply  daily  for  30  days. 

2.  Heat  value  chart  showing  total  supply  monthly  for  12  months. 

3.  Heat  absorption  showing  total  supply  daily  for  30  days. 

4.  Heat  absorption  showing  total  supply  monthly  for  12  months. 

5.  Heat  chart  by  several  furnaces  factors  shown  graphically. 

6.  Heat  chart  by  several  boiler  setting  factors  shown  graphically. 

7.  Heat    chart    by    several    breeching    and    stack    factors    shown 

graphically. 

8.  Heat  reclaimed  from  exhaust  shown  graphically. 

9.  Heat  delivered  to  engines  after  subtracting  B.E.  auxiliaries. 
10  4  Heat  delivered  to  switch  board  as  electric  energy. 

11. 

12.  Chart  showing  combination  of  all  above  month  by  month. 

XVI.  Cost  of  Power  Production  on  K.W.H.  basis: 

1.  Coal 

2.  B.E.  Labor. 

3.  B.K.  Maintenance. 

4.  Engine  Eoom  Labor. 

5.  Engine  Boom  Maintenance. 
6. 

7. 
8. 
9.  Chart  combining  all  XVI  items  month  by  month. 


INDEX 


Air,  composition  of,  2 

leakage  in  ducts,  152 

percent  excess  required,  7 

required  for  combustion,  (table), 
4 

spaces  in  boiler  walls,  185 

supplied  for  combustion,  3 

volume  required  per  developed 

horsepower,  (table),  148 
Alabama  coals,  95-118 
Altitude  corrections  for  fans,  (table), 

151 
Altitude     corrections     for     stacks, 

(table),  138 
American     Gas     &     Electric     Co., 

Windsor  plant,  202 
American  stoker,  30 
Analyses  of  coals,  proximate,  88 

table,  99 

ultimate,  89 
Anthracite,  culm,  97-125 

Colorado,  121 
Application  characteristics  of  stokera, 

162 

Application  of  stokers,  211 
Arches,  168 

chain  grates,  162-169 

overfeed  stoker,  165-169 

sprung,  170 

supsended, 163-169 

underfeed  stoker,  166-169 
Arkansas  coals,  analyses,  95-99 

characteristics,  119 
Ash,  sensible  heat  in,  14 
Ash  pits,  chain  grate  stokers,  193 

overfeed  stokers,  178 

underfeed  stokers,  193 
Auxiliaries,     (stoker)    engines,     153 

fans,  150 

turbines,  152 


Babcock  &  Wilcox  stoker,  34 
Baffles,  restriction  of,  140,  233 
Bodmer  stoker,  23 
Boiler,  boiler  room  log,  248 

draft  loss  through,  140 
Bone  coal,  analyses,  97 

characteristics,  128 

type  of  stoker  to  burn,  128 
Boston  Edison  plant,  187 
Breechings,   analyzing   losses,    136- 
144 

losses,  136 

Brightman  stoker,  27 
Brunton  stoker,  23 
Buffalo  General  Electric  Co.,  Niagara 

River  Station,  195 
Burke  stoker,  37 


Carbon,  burning  of,  1 

dioxide,  3 

Central  Station  plants,  stoker  equip- 
ment installed,  187 
Chain  grate  stoker,  34 

Babcock  &  Wilcox,  34 

Green,  39 

Illinois,  42 

Laclede-Christy,  44 

Playford,  50 

Westinghouse,  52 

Chimneys,    capacity,    sizes,  (table), 
133 

corrections  for  altitude,  138 

draft  table,  134 

formula  for  sizes,  132 

friction  loss,  (table),  132 

temperature  of  gases,  131 
Cleveland  Electric  Illuminating  Co., 
Lake  Shore  Station,  196 


253 


254 


INDEX 


Clinkering  of  coal,  81-172 
adhesions  to  walls,  173 
methods  to  avoid,  173 
Coal,  caking,  81 

changes  in  weight  and  volume 

by  wetting,  98 
characteristics  of,  106 
classifications  of,  89 
clinkering  of,  81 
coking,  81 

commercial  classification,  94 
anthracite  culm,  97-125 
bone  coal,  97-128 
Colorado,  95-121 
Eastern  bituminous,  94-108 
Eastern  Kentucky,  Tennes- 
see, and  Alabama,  95-1 18 
Middle  West,  94 
North  Dakota  lignite,  96- 

124 

Pittsburgh,  94-110 
Texas,       Oklahoma      and 

Arkansas,  95-119 
Washington,  Wyoming,  and 

Montana,  96-124 
composition  of,  88-93 
conservation,  32 
definitions,  80 
dust  in  coals,  139 
heating  value,  89-99 
Kent's  classification,  92 
producing  states,  87 
proximate  analysis,  88 
ultimate  analysis,  89 
uses  of,  82 
used  by  Central  Station  plants 

in  United  States,  99 
U.  S.  geological  survey  classifi- 
cation, 89 

U.  S.  production,  84 
World's  production,  84 
World's  reserve,  83 
Coal  hoppers,  capacity  of,  225 
Coke  breeze,  analyses,  96 

characteristics,  127 
Colorado     coals,     analyses,     95-99 

characteristics,  121-123 
Combustion  characteristics  of  coal, 
106 


Combustion,  principles  of,  1 

temperature  of,  8 
Combustion  rates,  108,  114,  116,  118, 

120,  122,    123,    125,    126 
chain   grate   stoker,    112,    113, 

121,  216 

overfeed  stoker,  108,  110,  113, 

115,    116,    118,    120,    122, 

123,  126,  128,  216 

underfeed  stoker,  111,  113,  114, 

115,  118,  119,  123,  126,  215 

Combustion     space,     chain     grate 

stoker,  181 
overfeed  stoker,  181 
underfeed  stoker,  179 
Commonwealth  Edison  Co.  North- 
west Station,  207 
Conditions    necessary    for    burning 

coal,  32 

Continental  stoker,  46 
Contract    forms,    used    by    stoker 

manufacturers,  244 
Consolidated    Gas    &    Electric  Co., 

Westport  Station,  194 
Coxe  stoker,  24,  37,  53 
Cox-Fulton  stoker,  53 


Dakota  coal,  analyses,  96 
characteristics,  124 
lignite,  124 
Dampers,  friction  losses,  141 

sizes,  141 

Detroit  stoker,  56-76 
Detroit  Edison  Co.,  Delray  Station, 

203 

Denver  Gas  &  Electric  Co.,  210 
Development  of  mechanical  stokers, 

31 
Draft,  129 

effects  on  brickwork,  178 
forced,  150 

formula  for  stacks,  132 
induced,  146 

losses,    through   fuel   bed,    137 
through  boiler,  140 
through  breeching,  141 
through  damper,  141 


INDEX 


255 


Draft,  through  stack,  142 
natural,  130 
required  for,  chain  grate  stoker, 

163 

overfeed  stoker,  138 
underfeed  stoker,  139 
Drake  fire  brick  blocks,  173 
Duquesne  Light  Co.,  Colfax  Station, 
199 


E 


Early  forms  of  stokers,  22 

Eastern  coals,  analyses,  94-99 
characteristics,  108 

Efficiency,    typical    capacity,    effi- 
ciency curve,  212 
of  chain  grate  stoker,  159 
of  combustion,  10 
of  overfeed  stoker,  158 
of  underfeed  stoker,  156 

Engineering  data,  required  for  stoker 
installations,  234 

Evaporation  chart,  220 


Fans,  corrections  for  altitude,  (table), 
151 

forced  draft,  150 

induced  draft,  148 
Feeding    of    coal    to    furnaces,    22 
Flexibility  of,   chain  grate  stokers, 
159 

overfeed  stoker,  158 

underfeed  stokers,  157 
Flue  gas,  analyses,  10 
Frederick  stoker,  71 
Frisbie  stoker,  29 
Fuel,  definition,  80 
Furnace  design,  168 


G 


Gases,  flue,  10 

mixture  of  furnace  gases,  177 

velocity,  (formula),  131 
Grate  areas  of  stokers,  chain  grate, 
223 

overfeed,  217 

underfeed,  223 
Grate   speeds,    chain    grate    stoker, 

161 

Green  stoker,  39 
Guarantees,  stoker  performance,  243 


H 


Hall  stoker,  25 

Harrington  stoker,  47 

Heat,    flow  through  furnace   walls, 

182 

loss  in  flue  gases,  10 
loss  in  unburned  gases,  11 
loss  due  to  superheated  steam, 

13 
Heating    value    of    coals,     Kent's 

table,  92 
table,  93-99 

Holyrod-Smith  stoker,  28 
Hydrocarbons,  1 


Illinois  coals,  analyses,  93-99 

characteristics,  113 
Illinois  stoker,  42 
Indiana  coals,  analyses,  93-99 

characteristics,  113 
Installation    of    stokers,    things    to 

provide  for,  226 
Iowa  coals,  analyses,  93-99 

characteristics,  115 


Jukes  stoker,  64-71 
K 


Gases,  density  of,  4 

dry  gas  per  Ib.  of  carbon,  (for- 
mula), 3  Kentucky    coals,    analyses,  93-94- 
dry  gas  per  Ib.   of  fuel,    (for-  95-99 

mula),  4  characteristics,  110-118 


256 


INDEX 


Labor  saved  by  mechanical  stokers, 

33 

Laclede-Christy  stoker,  44 
Lignite,  Dakota,  characteristics  of, 

93-96-99-124 

Denver,  characteristics  of,  121 
Texas,  characteristics  of,  92-95 


M 


McDougal  stoker,  25 
McKenzie  stoker,  34-47 
Mechanical  stokers,  16 
Merchants  Heat  &  Light  Co.  station, 

206 
Michigan  coal,   analyses  of,   93-99 

characteristics  of,  112 
Middle  West  coals,  94 
Minneapolis   General  Electric   Co., 

Riverside  station,  208 
Missouri  coals,   analyses  of,   93-99 

characteristics  of,  113 
Mixture    of    gases,    methods,     180 

requirements  for,  177 
Model  stoker,  54 
Moloch  stoker,  73 

Moisture  in  coals,  effect  on  combus- 
tion, 106 
Montana    coals,    analyses    of,    96 

characteristics  of,  124 
Murphy  stoker,  26-34-55 


N 


Overfeed  stokers,  Detroit,  34-56 

Hall,  25 

McDougal,  25      . 

Model,  54 

Murphy,  26-34-55 

Roney,  27-34-58 

Vicars,  25 

Wetzel,  59 

Wilkinson,  28-61 
Oxygen,  2 


Pennsylvania  coals,  analyses,  94-99 

characteristics,  106-110 
Philadelphia  Electric  Co.,  Delaware 

Ave.  station,  192 
Pittsburgh  coals,   analyses  of,   93- 

94-99 

characteristics,  110 
Playford  stoker,  50 
Present  types  of  stokers  in  U.  S.,  34 
Principles  of  combustion,  1 
Proximate  analyses,  88 
Public  Service  Electric  Co.,  Essex 

Station,  180 

R 

Radiation  losses,  15 
Riley  stoker,  62 
Roach  stoker,  78 
Roney  stoker,  58 


Nitrogen,  2 


Ohio  coals,  analyses,  93-94-99 

characteristics,  110 
Oklahoma  coals,  analyses  of,  99 

characteristics,  119 
Oregon  coals,  analyses  of,  93-99 

characteristics,  124 
Overfeed  stokers,  25 

Brightman,  27 

Cox-Fulton,  53 


Selection  of  stoker  equipment,  106- 
154 

application  conditions,  162 

draft  conditions,  162 

load  conditions,  155 
Sizes  and  dimensions  of  stokers,  211 

chain  grate  stokers,  223 

coal  hoppers,  224 

overfeed  stokers,  218-222 

underfeed  stokers,  221-223 
Smoke  prevention  devises,  historical, 

Barber,  20 

Gray,  19 


INDEX  257 

Smoke  prevention  devices,  Greyson,  Stokers,  Stowe,  34,  51 

17  Sturtevant,  34 

Langen,  20  Taylor,  30,  34,  66 

Predeaux,  20  Type,  "E",  34-74 

Robertson,  16  Vicars,  25 

Rodda,  19  Weller,  24 

Shanter,  19  Westinghouse,  34,  52,  68,  224 

Wakefield,  17  Wetzel,  34-59 

Watt,  16  Wilkinson,  28-61 

Williams,  20  Stowe  stoker,  51 

Witty,  18  Sturtevant  stoker,  34 

Smoke  abatement,  31  Sulphur  in  coal,  2 

Soot  deposited  in  gas  passages,  14  effect  on  clinkers,  81 
Specific  heat  of  flue  gases,  8 

Specifications    for    stokers,    typical  T 

specification,  236 

Speed  of,  chain  grate,  158-161  on  ox  AA 

verfeed   59  Taylor  stoker,  30-34-66 

j    .,  «-  Temperature,  of  combustion,  8 

underfeed,  167  f  f              Q 

Stoker  equipment  of  modern  steam  ol  lurnace>  y 

Tennessee  coals,  analyses  of,  93-95- 
plants,  186 

Stokers,  mechanical,  American,  30  . 

„   '        IP  1*7-1         o/i  characteristics,  118 
Babcock  &  Wilcox,  34 

Texas   coals,    analyses   of,    93,    95, 

Bodmer,  26  __ 

Brightman,  27 

'  characteristics,  119 

Brunton  23  Traveling  grate  stokers,  23 

Burke,  34-37  _    ,e  '      „_ 

,     A  Bodmer,  73 

Continental,  46  00 

„            ',      '  Brunton,  23 

Cox-Fulton,  53  ' 

BurKe,  d/ 


7fi  Continental,  46 
84,  56,  76  37 

Fnsbie,  29  TT      .  0/1    .« 

Green,  34-39  Hamngton,  34,  47 

Hall    2-i  JU   B8' 


Hdyrod-Srnith^S 

Ilhno,s  34-42  „  gtoke 

H 


. 

Laclede-Christy,  34-44 
McDougal,  3-25 
McKenzie,  34-47 

Model,  34-54  Ultimate  analyses,  89 

Moloch,  73  Underfeed  stokers,  28 

Murphy,  25,  34,  55  American,  30 

Playford,  34-50  Detroit,  34^56-76 

Riley,  34,  62,  221  Frederick,  71 

Roach,  34-78  Frisbie,  29 

Roney,  27,  34,  58,  218,  222  Holyrod-Smith,  28 

17 


258 


INDEX 


Underfeed    stokers,   Jones,    29-34- 
64-71 

Jukes,  28 

Moloch,  73 

Riley,  34-62-221 

Roach,  34,  78 

Taylor,  30,  34,  66 

Type  "E,"  34,  74 

Westinghouse,  34,  52,  68,  224 
Union  Electric  Light  &  Power  Co., 

207 
United  Electric  Light  &  Power  Co., 

Hellgate  plant,  189 
United    Gas    &    Electric    Co.,    205 
Use  of  coals,  82 


Virginia  coals,  analyses,  94 

characteristics,  106 
Volatile  matter,  88 
Volume  of  air  per  developed  boiler 
horsepower,  148 


W 


Washington  coals,  analyses  of,  46, 

93,  99 
characteristics,  124 

Weights  of,  products  of  combustion, 

(table),  7 

of  coals,  (table),  104 
stokers,  62,  66 
Weller  stoker,  24 
Westinghouse  stoker,  52,  68 
West  Penn  Power  Co.,  Springdale 

plant,  201 
West   Virginia   coals,   analyses,   93, 

94,  99 
characteristics,  106-110 

Wetzel  stoker,  59 

Wilkinson  stoker,  61 

Wind      box      pressure,      underfeed 

stokers,  139 
Wyoming  coals,  analyses  of,  46,  93, 

99 
characteristics,  124 


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